CHAPTER 1
INTRODUCTION
Whenever there is technology, energy is concerned and looked at as the prime mover.
Since this is a technological world therefore energy and its distribution is the cause of
almost all our problems. That is why most country economies are attributed to the
countries per-capita annual consumption of energy.
In this current world energy crisis, Kenya like most of the developing countries is still
relying on the prospects of fossil fuels. Therefore we are still relying on foreign imports
of fuel which in return depletes the little foreign exchange the country earns.
This situation is further worsened by the already exhausted wood reserves (take Mau
forest as an example) which are the readily available source of energy for most of the
national population composed of almost 80% of the population. This also has a direct
negative effect on the countries climatic changes, considering all these we are compelled
to look for an alternative source of energy. The immediate solution is solar energy.
The concept of solar energy is not a new one but its various applications are, this is due to
the unpopularity of its applications resulting from the previously accomplished sources of
energy like fossil fuels and wood fuels. However, due to the world’s awareness of fossil
fuel pollution, most countries and people are turning to harvest it both in large and small
scale.
Solar energy is commonly easily harnessed in the tropic regions because of the longer
sunshine experiences compared to non-tropic regions. Kenya is luckily within the tropics
thus an advantage to its population. Even with all the luck, the main problem comes in
the collection and conservation of this renewable energy plus the high and prohibiting
initial costs of capital to set even a small scale solar station. But the most promising part
of it all is the continued decrease in prices of solar tapping equipments over time as more
technological improvements are done by our able scientists.
1
Except for this, more still needs to be done to propel solar energy to its rightfully desired
position, position one! This project was thereby decided upon so as to inculcate these
noble ideas into practice.
This project of designing a solar powered domestic lawnmower that utilizes solar power
as an energy source is meant to address a number of issues that standard internal
combustion engine mowers do not. An electric lawnmower with a solar charger will be
easier to use. It will eliminate those unnecessary trips to the gas station for fill-ups. Just
plug the mower into the charging station when not in use and it will be charged and ready
for your next mow!
Most importantly it eliminates the emissions of an internal combustion mower which are
mostly responsible for environmental pollution and causes the green house gases effect
believed to be responsible for the worsening global warming of our planet. This is so
because solar energy is green/renewable energy.
The basic idea is to convert an older non-working gas mower into an electric powered
mower by replacing the gas engine with an electric motor that runs from a 12 volt battery.
This battery will be charged using a photovoltaic solar panel.
Due to cost, we chose to convert an old gas mower rather than just starting designing and
building one from scratch. This enables us to design from the power output. Also we are
to recycle as many material parts as we can. This will help to save these materials from
ending up in our already over filled dumpsites thus reducing pollution. Most of the design
materials used are already readily available in the local market in standard form. These
include;
The 12V battery.
The solar panel, 40 watts 17 volts.
The Charge controller.
The Dc circuit breaker.
The electric Dc motor
Old lawn mower deck
2
1.0
ENERGY DEMAND IN KENYA
Kenya’s energy scene is dominated by two key factors; an over-reliance on dwindling
biomass energy resource to meet the energy needs of most of the rural households, and a
heavy dependence on imported petroleum which drives the needs of the modern
economy. Traditional biomass is the main source of primary energy for the majority of
the population and accounts for up to 68% of the total energy consumed. Petroleum and
its related products, though mainly used by the modern sectors of the economy, accounts
for 22% of the total energy consumed. Use of electricity is usually associated with a rise
or change in lifestyle and its consumption accounts for a modest 9%. There are other
renewable energy sources which though used, are not widely disseminated. In spite of the
increased demand for energy in the country, dissemination and widespread use of modern
energy sources is still very limited. The current installed capacity in Kenya is about 1,197
MW. The number of electrical connections, which is a sign of the growing power sector
in the country, has risen drastically over the years from a low of 265,413 customers in
1990 to a high of 802,249 customers in 2006. Nevertheless, electrification levels in
Kenya are still very low with the rural household electrification levels averaging less than
5%.
1.1
Macro-economic and energy sector overview
Over the last ten years the economy in Kenya has experienced large fluctuations but it
remains one of the largest in sub-Saharan Africa. Towards the end of the 1990s it
experienced a large downturn to the extent of achieving a near zero growth in 2002.
Thereafter, there was a steady rise up to 2007. This period of steady growth reflected the
incoming of a reformist government which is largely in place up to the present. A major
part of the national import bill goes towards purchase of petroleum as the country has no
known oil resources.
Other local sources of commercial energy are electrical power generated from
hydroelectric stations, geothermal wells, and other renewable energy resources.
3
Electricity generated from oil products is also used for supplementation of power from
the local resources particularly during droughts. These electric power sources and their
contributions to the total electricity capacity are indicated in Table 1 below
Table 1 (electric power sources and their contributions to the total electricity capacity)
Ref.1
THERMAL
HYDRO
GEO-
WIND
BIOMASS
TOTAL
(%)
907
85.8
5.1
0.48
2
145
13.7
1057.1
THERMAL
KENGEN
134.6
657
115
0.4
KPLC
5.1
IPPs
130
Totals
269.7
657
128
0.4
2
(%)
25.5
62.2
12.1
0.04
0.2
13
For most rural people, who constitute about two-thirds of the national population the
dominant source of energy is traditional biomass in the form of fuel wood and charcoal.
The fuels are mainly used for cooking, and it is estimated that over 65% of the total
national energy consumption is derived from this source. This fact is a cause for concern
in view of the depletion of forests and consequent environmental degradation arising
from cutting of trees for fuel wood and burning of charcoal.
1.2
Evolution of energy policy and legal framework
Even as the economy has been going through a “seesaw” trend, the energy sector has
been on a continuous change path in an effort to improve the sector’s efficiency.
Energy policy development was initiated in 1987, but momentum picked up in the power
sub-sector when unbundling was done in 1997 to create separate national generation and
T&D (Transmission and Distribution) utilities.
4
It is further indicated that, as part of the unbundling, independent power producers along
with KENGEN (the public power producer), were allowed to generate power for sale to
the T&D utility (KPLC). Simultaneously, an Electricity Regulatory Board (ERB) was
formed to regulate the newly constituted power industry. These changes were embodied
in legislation by the Electricity Power Act, 1997. Subsequently, a major policy change
affecting the whole energy sector took place when the reform started in 1987, and
culminated in the 2004 Energy Policy.
. The key features of the policy, which is being implemented up to now, include the
conversion of the ERB into ERC (Energy Regulatory Commission) which regulates the
whole energy sector. The policy through the formation of a Rural electrification (RE)
authority, provides specific funding for RE, and promote development of RE by various
actors including private sector developers. The policy also aims at accelerating the
adoption of renewable energy as a means of improving energy security and access to
modern energy.
1.3
Regulatory provisions and tariffs
The principal functions of the ERC are licensing of bodies and individuals interested in
the provision of energy and energy services; tariff setting and approval; and protection of
rights and privileges of energy providers, consumers, and all parties providing services
for support of energy supply and utilization. Within the scope of these functions, the
design and approval of power purchase agreements (PPAs) have taken a central position
as the number of power producers and sellers is rapidly increasing. In this respect, the
commission has had to create model PPAs for use by small operators who have limited
financial means to obtain legal services for tailor-made PPAs.
As part of the ERC’s mandate to ensure fair play and stimulate growth of certain aspects
of the energy sector it has become critical to have the right tariffs planned and
implemented.
5
1.4
Rural electrification and renewable energy development
According to the Ministry of Energy, provision of electric power to rural areas, as a
separate programme from the scheme of supply for urban and peri-urban locations, was
started in 1973. The programme (REP) was originated and funded by the government
through its own finances and assistance from donors, but its implementer is KPLC. The
situation is now changing after the creation of the Rural Electrification Authority (REA),
which takes over the mandate of running the programme. It is expected that under the
REA the development of rural electrification will be enhanced, as the REA can give more
attention to the REP than KPLC has done. As evident from table 2, KPLC has been able
to do much better in supplying urban and other non-rural areas.
Table 2: KPLC (public) electricity consumers Ref.1
Separately from the REP, there has been a private form of rural electrification effort by
Entrepreneurs and individual groups.
The most common type of electrification in this category is solar photovoltaic (PV)
power supply that is aggressively marketed by solar power businesses. There has been
little intervention from government in the industry apart from removal of import duty for
solar panels.
6
It is estimated that close to 200,000 PV power installations have so far been put up
around the country. Then there are a growing number of rural community groups that are
undertaking projects to supply themselves with power from renewable energy sources.
While NGOs and donor agencies have been supporting such groups financially and with
technical assistance, there is significant contribution by the groups themselves. On the
whole, the private form of rural electrification has been carried out with a bottom-up
approach, and has manifested itself as the way forward for sustainably meeting the
modern energy needs of rural people in the country. The importance of renewable energy
other than from large hydro is recognized in respect of improving access to energy. This
specific endorsement is key to the development of renewable energy resources, and to the
provision of sustainable energy in rural areas. Kenya recently introduced a feed-in tariff
policy for wind, biomass and small-hydro resource generated electricity. The policy is
expected to boost exploitation of abundant local renewable energy sources in the country
by attracting private sector capital investments. The policy document defines the predetermined tariffs for both firm and non-firm power, with a more attractive tariff offered
for firm power. In addition, the policy defines a window for accessing these initial feed-in
tariffs, which would be applicable for the first 100MW of firm small hydro power and
50MW of nonfarm small hydro power; the first 150MW of wind power; and the first
150MW of firm biomass power and 50MW of non-firm biomass power.
(Source: Ref 10).
1.5
Consumer Satisfaction Index (CSI)
Consumer satisfaction survey was done for petroleum products, electricity and renewable
energy providers. The analysis was based on six key variables; customer expectation,
perceived quality, perceived value, image, loyalty and handling of customer complains.
The causality model indicated that all variables had a significant impact on consumer
satisfaction. However, the most important factor on consumer satisfaction is perceived
quality which depends on customer expectation. Customer expectation and perceived
value had less impact, on customer satisfaction. The study finds that complaint was a
very important factor in the biomass sector. Petroleum products providers’ customer
satisfaction was mostly influenced by perceived quality followed by image and perceived
7
value. Loyalty depended upon customer satisfaction and image. All variables had a
significant impact on consumer satisfaction for electricity providers. However, the most
important factor on consumer satisfaction was perceived quality.
In biomass, all latent variables had average scores ranging from 52% to 62%, with the
level of consumer satisfaction being comparatively higher than of the other factors.
Perceived value and customer loyalty had the highest and second highest average
implying that the focus on the short term strategy should be on quality of products and
services as well as fees and prices.
Table 3; Consumer Satisfaction Index (CSI): summary of key variables Ref.1
8
CHAPTER 2
LITERATURE SURVEY
2.0 SOLAR ENERGY REVIEW
In today's climate of growing energy needs and increasing environmental concern,
alternatives to the use of non-renewable and polluting fossil fuels have to be investigated.
One such alternative is solar energy.
Solar energy is quite simply the energy produced directly by the sun and collected
elsewhere, normally the Earth. The sun creates its energy through a thermonuclear
process that converts about 650,000,000 tons of hydrogen to helium every second. The
process creates heat and electromagnetic radiation. The heat remains in the sun and is
instrumental in maintaining the thermonuclear reaction. The electromagnetic radiation
(including visible light, infra-red light, and ultra-violet radiation) streams out into space
in all directions.
Only a very small fraction of the total radiation produced reaches the Earth. The radiation
that does reach the Earth is the indirect source of nearly every type of energy used today.
The exceptions are geothermal energy, and nuclear fission and fusion. Even fossil fuels
owe their origins to the sun; they were once living plants and animals whose life was
dependent upon the sun. Much of the world's required energy can be supplied directly by
solar power. More still can be provided indirectly.
Due to the nature of solar energy, two components are required to have a functional solar
energy generator. These two components are a collector and a storage unit. The collector
simply collects the radiation that falls on it and converts a fraction of it to other forms of
energy (either electricity and heat or heat alone). The storage unit is required because of
the non-constant nature of solar energy; at certain times only a very small amount of
radiation will be received. At night or during heavy cloud cover, for example, the amount
of energy produced by the collector will be quite small.
9
The storage unit can hold the excess energy produced during the periods of maximum
productivity, and release it when the productivity drops.
In practice, a backup power supply is usually added, too, for the situations when the
amount of energy required is greater than both what is being produced and what is stored
in the container.
Methods of collecting and storing solar energy vary depending on the uses planned for
the solar generator. In general, there are three types of collectors and many forms of
storage units.
The three types of collectors are flat-plate collectors, focusing collectors, and passive
collectors. Flat-plate collectors are the more commonly used type of collector today.
They are arrays of solar panels arranged in a simple plane. They can be of nearly any
size, and have an output that is directly related to a few variables including size, facing,
and cleanliness. These variables all affect the amount of radiation that falls on the
collector. Often these collector panels have automated machinery that keeps them facing
the sun. The additional energy they take in due to the correction of facing more than
compensates for the energy needed to drive the extra machinery.
Focusing collectors are essentially flat-plane collectors with optical devices arranged to
maximize the radiation falling on the focus of the collector. These are currently used only
in a few scattered areas. Solar furnaces are examples of this type of collector. Although
they can produce far greater amounts of energy at a single point than the flat-plane
collectors can, they lose some of the radiation that the flat-plane panels do not. Radiation
reflected off the ground will be used by flat-plane panels but usually will be ignored by
focusing collectors (in snow covered regions, this reflected radiation can be significant).
One other problem with focusing collectors in general is due to temperature. The fragile
silicon components that absorb the incoming radiation lose efficiency at high
temperatures, and if they get too hot they can even be permanently damaged. The
focusing collectors by their very nature can create much higher temperatures and need
more safeguards to protect their silicon components.
10
Passive collectors are completely different from the other two types of collectors. They
absorb radiation and convert it to heat naturally, without being designed and built to do
so. All objects have this property to some extent, but only some objects (like walls) will
be able to produce enough heat to make it worthwhile. Often their natural ability to
convert radiation to heat is enhanced in some way or another (by being painted black, for
example) and a system for transferring the heat to a different location is generally added.
People use energy for many things, but a few general tasks consume most of the energy.
These tasks include transportation, heating, cooling, and the generation of electricity.
Solar energy can be applied to all four of these tasks with different levels of success.
Heating is the business for which solar energy is best suited. Solar heating requires
almost no energy transformation, so it has a very high efficiency. Heat energy can be
stored in a liquid, such as water, or in a packed bed. A packed bed is a container filled
with small objects that can hold heat (such as stones) with air space between them. Heat
energy is also often stored in phase-changer or heat-of-fusion units. These devices will
utilize a chemical that changes phase from solid to liquid at a temperature that can be
produced by the solar collector. The energy of the collector is used to change the
chemical to its liquid phase, and is as a result stored in the chemical itself. It can be
tapped later by allowing the chemical to revert to its solid form. Solar energy is
frequently used in residential homes to heat water. This is an easy application, as the
desired end result (hot water) is the storage facility. A hot water tank is filled with hot
water during the day, and drained as needed. This application is a very simple adjustment
from the normal fossil fuel water heaters.
Swimming pools are often heated by solar power. Sometimes the pool itself functions as
the storage unit, and sometimes a packed bed is added to store the heat.
Whether or not a packed bed is used, some method of keeping the pool's heat for longer
than normal periods (like a cover) is generally employed to help keep the water at a warm
temperature when it is not in use.
11
Solar energy is often used to directly heat a house or building. Heating a building requires
much more energy than heating a building's water, so much larger panels are necessary.
Generally a building that is heated by solar power will have its water heated by solar
power as well. The type of storage facility most often used for such large solar heaters is
the heat-of-fusion storage unit, but other kinds (such as the packed bed or hot water tank)
can be used as well.
This application of solar power is less common than the two mentioned above, because of
the cost of the large panels and storage system required to make it work. Often if an
entire building is heated by solar power, passive collectors are used in addition to one of
the other two types. Passive collectors will generally be an integral part of the building
itself, so buildings taking advantage of passive collectors must be created with solar
heating in mind.
These passive collectors can take a few different forms. The most basic type is the
incidental heat trap. The idea behind the heat trap is fairly simple. Allow the maximum
amount of light possible inside through a window (The window should be facing towards
the equator for this to be achieved) and allow it to fall on a floor made of stone or another
heat holding material. During the day, the area will stay cool as the floor absorbs most of
the heat, and at night, the area will stay warm as the stone re-emits the heat it absorbed
during the day. Another major form of passive collector is thermos phoning walls and/or
roof. With this passive collector, the heat normally absorbed and wasted in the walls and
roof is re-routed into the area that needs to be heated. The last major form of passive
collector is the solar pond. This is very similar to the solar heated pool described above,
but the emphasis is a warm pool. With the solar pond, the whole purpose of the pond is to
serve as an energy regulator for a building.
Solar energy can be used for other things besides heating. It may seem strange, but one of
the most common uses of solar energy today is cooling. Solar cooling is far more
expensive than solar heating, so it is almost never seen in private homes.
12
Solar energy is used to cool things by phase changing a liquid to gas through heat, and
then forcing the gas into a lower pressure chamber. The temperature of a gas is related to
the pressure containing it, and all other things being held equal, the same gas under a
lower pressure will have a lower temperature.
Besides being used for heating and cooling, solar energy can be directly converted to
electricity. Most of our tools are designed to be driven by electricity, so if you can create
electricity through solar power, you can run almost anything with solar power. The solar
collectors that convert radiation into electricity can be either flat-plane collectors or
focusing collectors, and the silicon components of these collectors are photovoltaic cells.
Photovoltaic cells, by their very nature, convert radiation to electricity. This phenomenon
has been known for well over half a century, but until recently the amounts of electricity
generated were good for little more than measuring radiation intensity. Most of the
photovoltaic cells on the market today operate at an efficiency of less than 15%; that is,
of all the radiation that falls upon them, less than 15% of it is converted to electricity. The
maximum theoretical efficiency for a photovoltaic cell is only 32.3%, but at this
efficiency, solar electricity is very economical. Most of our other forms of electricity
generation are at a lower efficiency than this.
Hope for bulk solar electricity should not be abandoned, however, for recent scientific
advances have created a solar cell with an efficiency of 28.2% efficiency in the
laboratory. This type of cell has yet to be field tested. If it maintains its efficiency in the
uncontrolled environment of the outside world, and if it does not have a tendency to
break down, it will be economical for power companies to build solar power facilities
after all.
Of the main types of energy usage is transportation. Large, relatively slow vehicles like
ships can power themselves with large onboard solar panels, electric cars that are
partially powered by solar energy are available now, but it is unlikely that solar power
will provide the world's transportation costs in the near future.
13
Solar power has two big advantages over fossil fuels. The first is in the fact that it is
renewable; it is never going to run out. The second is its effect on the environment.
While the burning of fossil fuels introduces many harmful pollutants into the atmosphere
and contributes to environmental problems like global warming and acid rain, solar
energy is completely non-polluting.
While many acres of land must be destroyed to feed a fossil fuel energy plant its required
fuel, the only land that must be destroyed for a solar energy plant is the land that it stands
on. Indeed, if a solar energy systems were incorporated into every business and dwelling,
no land would have to be destroyed in the name of energy. This ability to decentralize
solar energy is something that fossil fuel burning cannot match.
As the primary element of construction of solar panels, silicon, is the second most
common element on the planet, there is very little environmental disturbance caused by
the creation of solar panels. In fact, solar energy only causes environmental disruption if
it is centralized and produced on a gigantic scale. Solar power certainly can be produced
on a gigantic scale, too.
Among the renewable resources, only in solar power do we find the potential for an
energy source capable of supplying more energy than is used. Solar is perhaps the most
promising. Numerically, it is capable of producing the raw power required to satisfy the
entire planet's energy needs. Environmentally, it is one of the least destructive of all the
sources of energy. Practically, it can be adjusted to power nearly everything except
transportation with very little adjustment, and even transportation with some modest
modifications to the current general system of travel. Clearly, solar energy is a resource
of the future.
(Source: Ref 2,7,9).
14
2.1
The Sun as a Source of Energy
Solar energy is energy derived from the heat and light from the Sun.
Several scientific works have concluded that the Sun is the central and the largest part of our
solar system, making up about 99.8% of the entire system.
It's mass is about 330,000 times the mass of the planet Earth and has a diameter of about 1.5
x 106 Km. It is about 1.5 x 108 Km away from the Earth and yet has a very powerful
influence on life on Earth.
The Sun's surface temperature is about 6,000oC, while the temperature of the core is about
1.4 x 107oC.
The temperature of the Sunspots (the coolest regions of the Sun's surface) is about 4,000oC
The luminosity (the amount of energy emitted by a star each second - Sun often considered
as a star) of the Sun is about 3.9 x 1026 Watts.
The Sun releases considerable amount of light and heat to the planets and stars of the solar
system. The Earth receives about 1.74 x 1017 W) of solar radiation from the Sun at the upper
atmosphere. Thirty percent (30%) of the solar radiation is believed to be reflected back
while the remaining seventy percent (70%) are absorbed by the atmosphere, oceans and land
masses. The absorbed energy is in the order of 3 - 4 x 1024 J per year. According to some
studies using the 2002 World's energy requirement figure, this absorbed energy figure
represents more energy in one hour than the Earth need in one year!
Literally, the sun powers the universe - everything on Earth derives its energy from the sun.
Over years, the sun energy is converted into other energy sources such as fossil fuels,
biomass, wind, hydropower etc which we have used extensively to meet the energy needs of
our World. These are indirect energy sources and one of them particularly; fossil fuels
(crude oil, gas and coal) have been used predominantly for energy supply to provide
electricity for light and heat for domestic and industrial uses. Unfortunately, fossil fuels
produce energy and produce with it considerable amount of pollutions and the greenhouse
gases that endanger our environment. Moreover, fossil fuels are not renewable.(Ref 7)
15
2.2 Solar Energy Applications
Solar energy has been used for thousands of years to dry clothes and agricultural products,
among other things. With the growing concerns over climate change and global warming
and the need for alternative energy sources, there is increased consciousness of using solar
for energy production for domestic and industrial uses.
The current and potential uses to which solar power can be put are:
•
Electricity generation through solar power plants or photovoltaic systems
•
Space heating and cooling in active and passive solar buildings;
•
Natural lighting or "day lighting";
•
Solar Water Heating and swimming pools;
•
Thermal cooking;
•
Water treatment by "thermal" distillation and disinfection;
•
Providing high temperature process heat for industrial purposes;
•
Agricultural purposes (drying, power supply for equipment, running greenhouses
etc);
•
Solar electrical vehicles.
The applications above can be classified under two broad headings:
(i) Solar Electricity; and
(ii) Solar Heating
2.3 Solar Energy and the Environment:
Solar energy is a renewable resource. A renewable resource is a resource that is able to be
replaced or replenished, either by the earth's natural processes or by human action. Solar
energy is available at varying proportions almost everywhere on earth. It cannot be depleted
unlike the fossil fuel based energy resources.
Solar energy is a “clean” energy resource. It does not involve the emission of Green House
Gases (GHGs) that are believed to be responsible for the worsening global warming of our
planet, Earth.
16
It provides a suitable energy alternative to the traditional fossil fuel energy sources that are
currently widely in use.
The slight drawback for Solar Energy is waste products generated from the use of silicon to
produce PVCs and possible desertification from operating solar thermal farms (expanse of
land containing collectors or PVCs). These defects can however be managed effectively to
limit impact on the environment.
2.4 Energy transfer
Heat naturally moves from warm to colder areas, always seeking to even out the
temperature of objects and substances in contact. Thos is made possible by the movement of
molecules. Molecules of hotter bodies move faster than those of cold ones hence they
transfer energy among themselves.
Solar energy –thermal energy- is transferred from the sun to the earth through the three
modes of heat transfer, mostly a combination of them. They are;
•
Conduction
•
Radiation
•
Convection
Conduction is the movement of heat through a solid substance. The density of the material
(molecular composition) affects the transfer. Denser materials transfer heat faster than less
dense ones.
Radiation transfer is through electromagnetic waves. This is achieved through heating the
target object maximally with minimum heating the medium of transfer. The object absorbs
radiant energy which it then radiates at a much longer wavelength than from the source
(sun).
Convection is the transfer of heat through air, water or other fluids. Heated fluid becomes
less dense and moves faster, colder fluid is heavier and displaces it upwards. Once it is up it
cools, gets denser thus displacing the warm layer beneath. The cycle repeats.
17
However trapped air must transfer heat by conduction thus it acts as an insulator, since air
molecules are further.
2.5 SOLAR ELECTRIC SYSTEMS
The parts involved in harnessing and changing solar energy to electric energy are as
follows:
•
Solar cell module
•
Charge controller
•
Battery
•
Wiring
•
The load
This chapter summarizes the role of each part in the system.
2.5.1 The solar cell module
Solar electricity is the direct conversion of sunlight to useful electricity. Whenever light
strikes solar cells they convert light energy to electric energy, this is done through a
principle of physics called photo-electric effect.
The basic unit of solar electricity conversion is the solar cell.
2.5.1.1 How solar cells work:
Solar radiation is comprised of millions of tiny high energy particles called photons. When a
photon of sufficient energy strikes a silicon atom in a solar cell, it pushes the outermost
silicon electron out of its orbit around the nucleus, freeing it to move across the cell’s
electric field.
The solar cell depends on the special electric property of the silicon element (or other semiconductor materials) which enables it to act as both a conductor and insulator.
Specially treated pieces of silicon “sort” electrons dislodged by solar energy in one direction
to produce an electric current. If a load is connected between the negative and positive side
of the cell, the electrons flow as a current.
18
Thus solar energy (in the form of photons) continuously dislodges silicon electrons from
their orbitals and “pushes” the electrons through the wires and load.
More intense sunlight gives stronger current. If the light stops striking the cell, the current
stops flowing immediately.
Solar cells vary in size and can be used according to the task at hand. No matter the task
though, all silicon type solar cells produce about 0.4 volts in normal operation. For this
reason, solar cells are connected in series to bring the voltage up to useful levels. Five cells
in series are enough to power a calculator of 2 volts, and 30-36 cells are enough to charge a
12 volt battery.
(Source: Ref 2).
2.5.2 The battery
Solar cell modules produce electricity only when the sun is shinning. They do not store
energy, therefore to ensure flow of electricity when the sun is not shinning, it is necessary to
store some of the energy produced. The most obvious solution is to use batteries, which
chemically store electric energy.
Batteries are groups of electro-chemical cells (devices that convert chemical energy to
electrical energy) connected in series.
Battery cells are composed of two electrodes immersed in electrolyte solution which
produce an electric current when a circuit is formed between them. The current is caused by
reversible chemical reactions between the electrodes and the electrolyte within the cell.
Batteries that are re-chargeable are called secondary or accumulator batteries.
As the battery is being charged, electric energy is stored as chemical energy in the cells.
When being discharged, the stored chemical energy is being removed from the battery and
converted to electrical energy.
In East-Africa, the most common type of secondary battery is the Lead-acid battery. As
indicated by its name, the lead-acid battery operates on the basis of chemical reactions
between a positive lead dioxide plate (PbO2), a negative lead plate (Pb) and an electrolyte
composed of sulphuric acid (H2SO4) with water (H2O).
On charging, PbO2 accumulates on the positive plate, Pb on the negative plate and the On
19
On charging, PbO2 accumulates on the positive plate, Pb on the negative plate and the
relative amount of sulphuric acid in the electrolyte increases. On discharging, lead sulphate
(PbSO4) accumulates on the negative plate and the relative amount of H2O in the electrolyte
increases. Each cell in a lead-acid battery has a voltage of about 2.0 volts; therefore a 12
volt battery has 6 cells connected in series.
←charging
Pb + PbO2 + 2H2SO4 ↔ 2PbSO4 + 2H2O
discharging→
The capacity of a battery is measured in amp hours (Ah). This indicates the amount of
energy that can be drawn from the battery before it is completely discharged. Note that Amp
hours is not a measure of energy. For example, a 100Ah battery should give a current of 2
amps for 50 hours.
Lead-acid batteries fall in two general categories: shallow discharge and deep discharge.
Deep discharge batteries are preferred for solar electric systems because more energy can be
taken out of them than shallow discharge without destroying the cells. Shallow discharge
batteries should not be discharged below 80% state of charge (this is a measure of the
energy remaining in a battery). Deep discharge batteries can be discharged up to a 40% state
of charge.
(Source: Ref 2).
2.5.3 The charge controller
The charge controller or control panel as it is commonly known has two primary functions.
First, it provides a central point for connecting the load, the module and the battery.
Secondly, it manages the system so that the harvested electricity is effectively used, and so
that components are protected from damage due to changing voltage levels.
The charge controller at the very least should act as a junction box. Here, the battery, load
and solar module are fastened together by means of connector strips.
20
Fuses are incorporated to protect the equipment from damage by short circuits.
Charge controllers contain a blocking diode. This blocking diode prevents current from
flowing from the batteries to the solar cell module when the battery voltage is higher than
the module voltage. This prevents energy losses in the system.
(Source: Ref 2).
Blocking diode
Switch
Battery
Load
Solar
module
Fig. 1: Charge controller circuit diagram
2.6 LAWN MOWERS ANALYSIS
A lawn mower is a device which by means of one or more revolving blades is used to cut
grass or other plants to an even length. Lawnmowers employing a blade that rotates about
a vertical axis are known as rotary mowers, while those employing a blade assembly that
rotates about a horizontal axis are known as cylinder or reel mowers. Many different
designs have been made, each suited to a particular purpose. The smallest types, pushed
by a human, are suitable for gardens, while larger, self-contained, ride-on mowers are
suitable for large lawns, and the largest, multi-gang mowers pulled behind a tractor, are
designed for large expanses of grass such as golf courses and municipal parks.
The first Lawn mower was invented by English engineer Edwin Beard Budding in 1827.
21
Budding’s mower was designed primarily to cut the lawn on sports grounds and
expansive gardens as a superior alternative to the scythe.
His patent of 25 October 1830 described "a new combination and application of
machinery for the purpose of cropping or shearing the vegetable surfaces of lawns, grassplats and pleasure grounds."
It took ten more years and further innovations to create a machine that could be worked
by donkey or horse power, and sixty years before a steam-powered lawnmower was built.
Around 1900, one of the best known English machines was the Ransomes' Automaton,
available in chain- or gear-driven models. JP Engineering of Leicester, founded after
World War I, produced a range of very popular chain driven mowers. About this time, an
operator could ride behind animals that pulled the large machines. These were the first
riding mowers. The rise in popularity of sports such as lawn tennis, cricket, football and
rugby helped prompt the spread of the invention. Lawn mowers became a more efficient
alternative to simply relying on gardeners wielding the scythe (which, when placed in
incompetent hands, left unsightly scars on and in the ground) or bare spaces caused by
domesticated grazing animals.
James Sumner of Lancashire patented the first steam-powered lawnmower in 1893. His
machine burned petrol as a fuel. After numerous advances, the machines were sold by the
Stott Fertilizer and Insecticide Company of Manchester and later, the Sumner's took over
sales. The company they controlled was called the Leyland Steam Motor Company.
Numerous manufacturers entered the field with gasoline-driven mowers after the turn of
the century. The first grass boxes were flat trays but took their present shape in the 1860s.
The roller-drive lawnmower has changed very little since around 1930. Gang mowers,
those with multiple sets of blades, were built in the United States in 1919 by a Mister
Worthington. His company was taken over by the Jacobsen Corporation but his name is
still cast on the frames of their gang units.
Rotary mowers were not developed until engines were small enough and powerful
enough to run the blades at a high speed. In the 1930s, Power Specialties Ltd. introduced
a gasoline-powered rotary mower. One company that produced rotary mowers
commercially was the Australian Victa company, starting in 1947. Early in the 1930s,
22
experiments in design of rotary mowing equipment were conducted by a farmer in the
Midwest region of the United States, by the name of C.C Stacy.
His concept was the use of a toothed circular saw blade mounted horizontally on a
vertical shaft, which would be suspended at a height of approximately 2" and moved
across a lawn to cut grass and other lawn vegetation at a uniform height. The power for
his experimental mower was an electric motor. The success of Stacy's design was limited
by 2 factors: the relatively small diameter of the saw blades he used for his experiments,
which were about 8"; and the fact that toothed circular saw blades, were not really an
optimum cutting tool for free standing grass and other plants. Stacy did not come up with
any idea for a cutter similar to modern rotary mower straight blades, and soon dropped
his experiments with rotary mowing. On May 9, 1899, John Albert Burr, an African
American inventor, patented [patent 624,749] an improved rotary blade lawn mower.
Burr designed a lawn mower with traction wheels and a rotary blade that was designed to
not easily get plugged up from lawn clippings. He also improved the design of lawn
mowers by making it possible to mow closer to building and wall edges. Lawn mowers
are classified according to the cutting mechanism it employs. There are two cutting
mechanisms in common use:
2.6.1 Reel or cylinder mowers;
These have a set of spiral-cylindrical blades spinning on a horizontal axis. Cutting is by a
scissor-like action between the moving spiral blades and a single stationary horizontal
blade, or "bed knife". The axle is attached to a gear that is then mounted on one of the
wheels in order to spin the blades rapidly for good grass cutting action even when the
mower is moving slowly.
2.6.2 Rotary mowers;
These have blades that spin horizontally on a vertical driveshaft. Cutting is due to a
horizontal blade striking the grass at a high speed.
The two cutting mechanisms can lead to different results. On rotary mowers, the blade is
usually not sharp enough to cut the grass cleanly. The speed of the blade simply tears the
grass resulting in ragged tips.
23
By contrast, the cylinder-type reel lawn mowers and manual lawn mowers usually work
by scissor action on the blades and a cleaner cut is achieved.
Rotary lawn mowers often allow the height of the lawn mower to be adjusted to control
the height of the cut grass. On older or less expensive lawn mowers, this is accomplished
by manually moving each wheel to a different slot on the chassis. A more recent
innovation in rotary mowers is a "one-touch" height-adjust mechanism where the blades
are mounted on a frame separate from the rest of the lawn mower and the frame can be
raised and lowered. On hover mowers, height adjustment is provided by the use of
removable spacing washers that fit between the blade and the motor spindle, since the
mower body must remain at the same height above the grass in order to preserve the air
cushion. Lawn mowers need power for two purposes: to cut and to move. The act of
pushing or pulling a reel mower provides power for cutting and moving at the same time.
For rotary mowers, the power sources may vary: grass-cutting may be powered by either
an internal combustion engine or an electric motor, while propulsion may share that
power source or be supplied by the user or another external source such as a tractor.
Wheel-driven gear systems allow for cutting to be powered by the same external source
as that used to propel the mower.
(Source: Ref 7,8).
2.7 TYPES OF LAWN MOWERS.
2.7.1 Cylinder Lawn Mowers.
Cylinder lawn mowers consist of a cylinder and a number of horizontal blades that spiral
around it. If adjusted to the correct height a cylinder mower will give the best cut of any
lawn mower. They are however, best suited to flat lawns and can be difficult to use on
bumpy or tall grass.
24
Fig. 2: Cylinder lawn mower. Ref 7.
The number of blades that spiral around the cylinder determines the quality of the finish,
the more blades, the better the cut. Cylinder mowers can also be fixed with a roller which
flattens the grass giving a smooth finish. A cylinder mower with a rear roller will give the
best striped effect.
Some cylinder mowers are entirely hand driven whereas others may have the blades
driven by an engine. Self propelled cylinder mowers use an engine to propel themselves
forward as well as to turn the blades. This can save the operator a lot of energy as little
effort is required to move the mower around the lawn.
2.7.2 Petrol lawn mowers.
These lawn mowers fall under the rotary type mowers. They are powered by internal
combustion engines. Such engines can be either two-stroke or four-stroke cycle engines,
running on gasoline or other liquid fuels. Internal combustion engines used with lawn
mowers normally have only one cylinder. Power generally ranges from two to seven
horsepower (1.5 to 5.25 kW).
25
Fig. 3: Petrol lawn mower. Ref 5
The engines are usually carbureted and require a manual pull crank to start them,
although an electric start is becoming a sales feature in some countries. These lawn
mowers typically have an opening in the side or rear of the housing where the cut grass is
expelled. Some have a grass catcher attachment at the opening to bag the grass clippings.
The key advantage of petrol lawn mowers is that they are both powerful, and highly
mobile. A basic petrol lawn mower will have at least 3hp where as an electric mower will
only have up to 2hp. The other advantage they have over electric mowers is that they
don't require a long power cable to be connected and dragged behind it. Depending on the
size of your lawn this could be quite an important issue.
Petrol lawn mowers come in both the 2 stroke and 4 stroke cycle engines;
•
2 stroke lawn mowers are more powerful, less expensive, have a better power to
weight ratio, and can be operated in any orientation. However, they are also
louder, less fuel efficient, and do not last as long due to the high revolving
engine. They also require the petrol to be mixed with oil prior to use in order to
lubricate the engine.
•
4 stroke lawn mowers by comparison are not as powerful, but contain an oil sump
(a separate oil chamber) and don't need the fuel to be pre-mixed. This saves the
operator from mixing the fuel but it also means that the mower cannot be tipped
on one side as it will cause the oil to fall out of the sump into the fuel chamber.
2.7.3 Electric lawn mowers.
Rotary mowers powered by electric motors are increasingly popular. Usually, these
mowers are moved by manual motive power the on-board engine or motor only spins the
blades. These have the disadvantage of requiring a trailing power cord that limits its
range and so these are only useful for relatively small lawns, close to a power socket.
There is the obvious hazard with these machines of mowing over the power cable, which
stops the mower and may put users at risk of electrocution. Installing a residual-current
26
device on the outlet can reduce the risk of electrocution.
Fig.4: Electric lawn mower. Ref 7
Electric rotary mowers weigh 45-50 pounds. The key advantage of an electric lawn
mower is the price. A basic rotary electric lawn mower will cost you about half the price
of a petrol mower, as well as being lighter, more quiet, and requiring less maintenance.
For a small lawn an electric mower should do fine, provided you mow regularly. If it is
any larger or if the grass tends to get out of hand then you will need the extra power of a
petrol mower.
2.7.4 Hover lawn mowers.
Hover mowers are powered rotary push mowers that use a turbine above the spinning
blades to drive air downwards, thereby creating an air cushion that lifts the mower off the
ground like a hovercraft. The operator can then easily move the mower as it floats over
the grass. Hover mowers can also be applied to very long grass and even light scrub,
since their lightness permits most operators to lift the mower up and then let it sink
slowly down while the blades progressively chop up the vegetation. The lifting action is
made even easier when the mower is swung around with the handle held against the
operator's mid-body to provide leverage. The action of air lifting the mower off the grass
makes it hover a few centimeters above the ground thus making it very easy to push
around the garden and, providing that you are not interested in getting stripes, means that
you can mow the lawn from side to side if you wish
27
.
Fig. 5:Hover lawn mower. Ref 7
Hover lawn mowers are cheap, light, and quiet. On the down side they are also quite low
power and so best suited to small lawns that are mowed often. They typically come in
electric but there are also petrol versions available.
2.7.5 Riding (ride-on) mowers.
A popular alternative for larger lawns is the ride-on mower. The operator is provided
with a seat and controls on the mower and literally 'rides' on the machine. Most use the
horizontal rotating blade system, though usually with multiple blades.
A common form of ride-on mower is the lawn tractor. These are usually designed to
resemble a small agricultural tractor, with the cutting deck mounted amidships between
the front and rear axles. An alternative layout for a ride-on is that with the mowing decks
in front of the machine, often called zero-turn mowers because of their maneuverability.
Mowing decks on ride on mowers are rotary and have two or three cutting blades. Deck
width can vary in size anywhere from 38” to 54” and wider for commercial mowers.
There is a wide range of brands on the market that offer various features and engine size.
A model with an engine output of 8.5HP would be suitable for a lawn up to one acre in
size. If your lawn is larger than that then you should consider a model that has 11.5HP to
13HP as they will not have to work as hard to do the job. These mowers are generally
more expensive.
28
Fig. 6: Ride-on mower. Ref 5
2.7.6 Robotic lawn mowers.
A typical robotic lawn mower requires the user to set up a border wire around the lawn
that defines the area to be mowed. The robot uses this wire to trim and in some cases to
locate a recharging dock. Robotic mowers are capable of maintaining up to 5 acres of
grass. Electricity usage varies from about 100 watts for 1/2 acre to 500 watts to maintain
5 acres. Robotic lawnmowers are increasingly sophisticated, are self-docking and contain
rain sensors nearly eliminating human interaction for mowing grass. Some models will
adapt their programming by detecting the speed in which grass grows as needed to
maintain a perfect cut lawn.
29
CHAPTER 3
DESIGN PROPOSAL
3.0 PRELIMINARY DESIGN.
3.1 Design Brief
3.1.1 Motor and battery sizing.
To begin with we need to design a mower that will fit our need and since it is for
domestic use, we estimate a mowing time of forty five minutes. Since it is electrically
powered we need a motor to run the blades and a battery to store the electric energy
tapped from the solar energy and to run the motor.
To size the battery accordingly we need to know the amperage the motor pulls for the
duration of time it will take to mow the grass. This will depend on the motor used and to
select the motor we compare the power output from a standard petrol lawn mower and
select a motor that will give the same power output.
Once we have selected the battery and the motor, we need to find out whether these will
meet our need by calculating the amp-hours needed to run this motor for the duration of
our mow.
3.1.2 Solar panel and charge controller sizing.
To charge the battery we need to have a solar panel to tap the solar energy from the sun
and convert it to electric energy that will be stored in the battery selected. We also need a
charge controller that will be wired between the solar panel and the battery. The work of
this controller is to stop the battery from discharging through the solar panel when there
is no sun and the panel is not creating electricity. It also prevents overcharging when the
battery reaches full charge and also regulates the voltage to the battery.
30
In selecting the solar panel we considered;
•
The average sun hours per day (insolation).
•
Size of the battery.
•
Current draw of the motor.
•
Frequency and duration of use of the mower.
3.1.3 Disassembling the Old Mower
An old petrol lawn mower is used and the engine is disassembled from the frame. This
frame is the one that we use to mount the motor and the battery for our new lawn mower
as shown in fig 7.
Fig.7: Stripped deck.
3.1.4 Deck fabrication
After stripping the mower and removing the engine, a huge hole is left on the deck. We
use a small sheet of stainless steel to cover the hole in the deck. Before welding the new
stainless steel sheet to the mower we drill a hole in the center to allow the shaft of the
motor to go through the deck. After drilling the hole the sheet is welded over the existing
hole in the mower. The stainless steel is also thick enough to support the motor.
3.1.5 Mounting the Motor.
The motor is mounted on the mower deck and it was ensured that it was centered and
properly secured. The motor is mounted to the deck with four bolts.
31
3.1.6 Mounting the blade.
This is the most important part and when designing for this, safety as a major factor was
put into consideration as the blade when in operation can be a safety hazard. Also the
weight of the blade and how to mount it on the motor shaft is also a key consideration.
Moreover the sharpness of the blade is another important aspect and this will depend on
the power and the RPM of the motor used.
3.1.7 Battery mounting
Once the motor is mounted we need to find the most suitable place to mount the battery
on our deck. A factor of weight distribution needs to be considered in determining on
where to mount the battery. To achieve this, the battery is mounted close to the rear
wheels and to make the mount firm it is supported to the base of the motor.
3.1.8 Electrical wiring.
A proper electrical wiring is to be put in place putting into consideration the safety of the
operator and any other person who may come into contact with the mower. To do this we
need to have the correct wire sizing, correct gauges and circuit breakers, switches and
terminals put in the right place.
Wiring need to be right since it is a key part of our lawn mower as through it the starting
and stopping of the mowers operations is achieved. Gauges such as the ammeter, the
voltmeter are used to indicate the power pulled by the motor from the battery and the
power remaining.
\3.1.9 Charging the battery.
After mowing the battery needs to be recharged to have enough power for use for the
next mowing session.
32
A proper wiring circuit for charging the battery needs to be put into considerations. We
need the right gadgets for this circuit, such as charge controllers, fuse, connectors,
ammeters and voltmeter to control the charging.
3.2 DESIGN
3.2.1 Mounting design.
The mounting in our design was considered to be the plate which is welded onto the
mower deck after removing the gas engine. We chose to design the mounting such that it
is one component to hold the battery and to mount the motor onto.
We use a flat stainless steel plate of 2mm thickness to cover this hole. This plate is cut to
a size bigger than the hole on the deck. The plate is then placed on the deck such that it
completely covers the hole and it is then spot welded. We determine the centre of the
deck and a hole of 16mm diameter is drilled through the plate. The hole is accurately
located at the centre so that the blade doesn’t touch the sides of the deck when in
operation. The remaining 2mm stainless steel plate is also used to fabricate for the battery
and motor mount.
On the stainless steel plate mark 30mm from the edges and mark a rectangle of the
battery dimensions. We cut the corners so as to fold the plate edges up at 900 to the
horizontal so as to form a tray like rectangular structure of the battery dimensions. One
edge is folded at 900up to a height of 300 mm and the corners are then welded together
for firmness.
The 300 mm plate is used to mount the motor whereby the base of the motor is bolted to
this plate. By measuring the height of the 4 holes on the motor base from the end of the
motor housing at the point where the shaft protrudes we drill 4 holes at the same heights
measured from the point at which the plate is folded, as those measured from the motor.
These holes are used to bolt the base of the motor onto the plate as a way of mounting it.
This structure is spot welded onto the deck such that the part for the battery mount is
placed near the rear wheels to ensure weight distribution by measuring the distance from
33
the centre of the motors shaft to its base such that the structure is spot welded at this
distance away from the centre of the 16 mm hole. This design is shown below in fig 8.
Fig. 8 mounting design
34
Its dimensions and placement are expected to hold the weight of the battery in place
which in turn aids in cancelling out the vibrations of the electric motor through a reaction
force presumed to be equal but opposite to the motor vibration force.
The complete assembly of the mower deck and mounting is expected to look as fig 9.
Fig.9: Fabricated mounting on the deck
3.2.2 The Electric Motor
The most popular domestic lawn mower is an internal combustion engine push mower
with a 5 horse power (HP) power rating. We require replacing this engine with an electric
motor of equivalent performance. Our research led us to the conclusion that a 1 HP
electric motor at peak power was almost equivalent to 4HP engine at peak power.
Motor specifications
•
12 volt direct current, ¾ HP electric motor was selected for the design.
•
This selected motor was specified to pull 58A (amperes) at full load.
•
The motor is set to run at a speed of 1400 revs per minute.
•
The shaft of the motor is of 15mm diameter and 50mm length.
35
The mounting technique used for mounting these motors is the base mounting. This
renders the motor a horizontal working direction which is not the case for a lawn mower.
Therefore for vertical working direction, the motor is mounted at the side of the lawn
mower using the designed mounting.
For the motor shaft to be at low enough distance relative to the mower deck, a spacer ring
is used. This ring is of about 5.0mm.
The right length of bolts and nuts (no extra extension through to the mounting but
sufficient to hold the motor tight without vibrations) were used. Leaving accurate
allowance for washers and split lock washers the motor is to be mounted firmly onto the
plate and lawn mower deck.
3.2.3 The Mower Blade
The blade is to be designed in such a way with high accuracy because it is essentially the
cutting tool of the mower; a picture of an ordinary gas lawn mower blade is shown
below;
Fig.10: Mower blade. Ref 7
Various critical factors governed by the motor speed were considered in choosing the
blade material, they include:
36
•
The weight of blade material
•
The strength of the blade material
•
The blade size in length
•
The blade size in thickness
•
Safety to the user
Apart from all these factors combined, the need for an easily sharpened material was also
considered in the case of maintenance. So as to achieve maximum efficiency from the
motor hence the whole lawn mower, the blade must be absolutely sharp. Sharpening is
done quite often by grinding.
We fabricate a blade from a 2mm thick stainless steel flat plate and 20mm wide. The
length of the blade should be less than that of the deck to avoid the blade touching the
sides of the deck during mowing. We then drill 3 holes one at the centre of the blade and
of diameter slightly greater than that of the motor at approximately 15.2mm.The other
two holes are of the same diameter as the threaded holes on the bushing collar and are at
equal but opposite distance from the centre.
The blade has to be mounted firmly to the motor shaft so as to guarantee maximum
safety and regular rotation. A balance on the blade has to be achieved such that there is
even rotation of blade i.e. a split taper bushing and a pulley is used to mount and align the
blade onto the motor shaft preventing vibration and excessive wear on the motor shaft
and bearings.
Split bushings have an advantage in the way they hold the pulley very firmly to the shaft
and can accommodate a lot of vibration. The bushings also have a slot to prevent
rotational slippage. The pulley helps in fastening the taper bushing to the shaft.
After fabricating the blade we bolt it onto the bushing collar using two bolts of the same
thread form to those of the bushing. Now the blade is firmly mounted on our mower.
A picture of a split taper bushing, pulley and bolts for this assembly is shown below;
37
Fig. 11: Taper bushing, pulley and bolts .Ref 8.
3.2.4 Battery
The electric motor selected was specified to pull 58Amps of electric current at full load.
The motor is being used to rotate a blade at high angular velocities to enable it to cut
grass. This action is not expected to load the motor but will put the motor at
approximately half load therefore the motor will only pull about 30Amps for its
operation. On average, the amount of time required to mow a lawn in a residential area is
about forty five minutes. The amp-hours pulled by the motor at this time become:
30 Amps * 45/60 hours = 22.5 amp-hours
A deep discharge battery can be drained up to a state of charge (SoC) of 0.4. The capacity
of the battery required can then be found as; (see appendix 1 for X)
(X-22.5)/X = 0.4
(X – 22.5) = 0.4X
X – 0.4X = 22.5
0.6X = 22.5
X (battery capacity) = 37.5Amps
A higher capacity battery is selected to provide a safety factor. We picked a 50Amp, 12
Volt deep discharge battery, which is readily available in the market. Using this battery
size the level of SoC on discharging is; (50-22.5)/50 = 0.55. This is an acceptable level.
38
3.2.5 Solar Panel and Charge Controller
The battery selected is a 50Amp, 12Volt deep discharge battery. To charge this fully, a
40watt, 17 volt solar panel would suffice. This panel has a resistance of 3 ohms this gives
an amperage of 17/3 = 5.7 amps. At this amperage a charge controller of 10 amps would
be appropriate.
3.2.6 Wiring
The wiring from the battery was done as shown in Fig 12
Fig. 12: Battery to motor circuit diagram
A 100A fuse between the battery and motor is used to protect the motor from short
circuit. An ammeter was connected in series, while a voltmeter was connected in parallel
with the battery. The ammeter helps to give an idea as to the amount of amps being
pulled when mowing the grass. This indicates as to how long the battery is going to last
before needing a charge. The voltmeter tells when to recharge the battery. The female
plug allows for connection to the panel (charging station).
39
3.2.7 The Charging Station
The charging station is like the refill station for our battery which is used as a source of
energy to drive the motor. This is done initially before using the battery for the first time
and subsequently thereafter each mowing session depleting its charge.
Fig. 13: Solar panel to battery circuit diagram
Importantly, since the solar panel and charge controller are of specified ratings, most of
our design went to the solar panel rack (holder).
3.2.7.1 The rack
This is the main solar panel safety device. It provides tight and safe holding reducing the
risk of a new panel. The design was done within our specific panel dimensions,
modifications can be done to accommodate other panel sizes. Some factors that were
taken into account included;
•
The base stands of the holder( firm to enable steady positioning)
•
The height from the ground, ( high to reduce obstacle interference )
•
The weight and strength of the materials used(for durability and mobility)
•
The corrosion resistance of the materials. (in corrosive environments).
•
Angular adjustment (for angular adjustments for maximum insolation )
•
Clamp locks( for locking the panel on rack)
40
We need an angle line steel bar 40mm in width from which we cut 4 pieces, two of
750mm length and the other two of 540mm length. The 4 pieces are welded together to
form a rectangular frame of dimensions 750mm by 540mm.The remaining angle plate is
used to fabricate the stand onto which the frame is mounted to. The stand is at a height of
700mm from the ground with four stands at an angle of 600 to the horizontal. They are to
be reinforced by a 4mm wide flat bar on all the four sides. The charge station is as shown
in figure 14.
Fig 14: The solar panel rack
41
3.3 PARTS LIST AND COSTING
Table 4: Parts list and cost
IMAGE
PART NAME
COST
(SHS)
An old gas lawn mower deck
Electric Motor (12V DC 3/4HP)
Battery (12V 50Amp Hour )
4000/=
6000/=
5500/=
Circuit Breaker. (40 to 80 Amp
Dc rated)
800/=
Split taper bushing and pulley
1000/=
Solar panel ( 40watts 17 volts)
12500/=
Charge controller
2900/=
Miscellaneous fabrication costs
6,500/=
Total cost
39,200/=
42
CHAPTER 4
4.1 DISCUSSION
After consideration of the current energy situation in Kenya, it was determined that the
harnessing of solar energy was a viable solution to the problem at hand. In the research
conducted on the various sources of energy utilized, renewable energy produced the
greatest customer satisfaction as is shown in table 3. However, renewable energy is not
yet fully tapped to its maximum potential.
Previous work on lawn mowers has seen the evolution from cylindrical to current rotary
cutting mechanisms. Rotary mowers have also moved from the carbon emitting and fuel
guzzling internal combustion engines to more environment friendly electrical mowers.
Nevertheless, these mowers are not common due to the long extension cords that are
characteristic of them. For this reason a solar lawn mower is seen as the perfect solution
as it is cordless and does not contribute to the electricity bill.
From the cost analysis of the design, an initial cost of Ksh.39,200/= is estimated. This is
a relatively high initial cost but less than the current market price of an internal
combustion engine lawn mower. Also, comparing the fuel consumption of the
combustion engine (at the current fuel price Ksh.115.10/=) and its cost of maintenance
with the running cost of the solar powered lawn mower, the project proposal is
considered feasible.
43
4.2 CONCLUSION
The objective of the project was to design a solar powered domestic lawn mower. This
was achieved by converting an internal combustion engine lawn mower to utilize solar
energy as its source of power, which was successful.
4.3 RECOMMENDATIONS
•
Further research to improve on our design by automation.
•
Fabrication of the solar powered lawn mower.
•
Further study on how to popularize usage of renewable energy should be done.
•
Adequate harnessing methods should be devised to achieve full potential of solar
energy.
•
Design of other solar powered equipment to ease the strain on fossil fuels and
electricity.
44
REFERENCES
1.
Annual reports and accounts 2007, Kenya Power and Lighting Company Limited.
http://www.kengen.co.ke
2.
Mark Hankins, Small Solar Electric Systems for Africa, Motif Creative Arts Ltd.
1991
3.
Agarwal M.P, Solar Energy, S.Chand & Company Ltd, New Delhi
4.
Home Power Magazine Company. Home Power Magazine.U.S.A
5.
Briggs & Stratton.
http://www.briggs&stratton.com
6.
Car & General.(K) Ltd.
Luska road/Dunga road Industrial area
7.
Wikipedia
http://www.wikipedia.com
8.
http://www.arttec.net
9.
Mark Hankins, Renewable Energy in Kenya, 1987
10.
Ministry of Energy, Energy consumption trends in Kenya
http://www.energy.go.ke
45
APPENDICES
46
Appendix 1
Table 1 (electric power sources and their contributions to the total electricity capacity)
THERMAL
HYDRO
GEO-
WIND
BIOMASS
TOTAL
(%)
907
85.8
5.1
0.48
2
145
13.7
1057.1
THERMAL
KENGEN
134.6
657
115
0.4
KPLC
5.1
IPPs
130
Totals
269.7
657
128
0.4
2
(%)
25.5
62.2
12.1
0.04
0.2
13
47
Appendix 2
Table 2: KPLC (public) electricity consumers
Appendix 3
1HP Electric motor ≈ 4 HP internal combustion engine.
48
Appendix 4
Table 3; Consumer Satisfaction Index (CSI): summary of key variables
49
Appendix 5
QUESTIONNAIRE
1) What are the different types of lawn mowers available in the market?.....................................
………………………………………………………………………………………………………
2) What are their power ratings?
……………………………………………………………………
3) What is their respective initial purchasing cost?..........................................................................
4) What is their respective cost of running and maintenance? …………………………………..
..................................................................... ……………………………………………………
5) Which is the most popular type of lawn mower in the market?
…………………………………………………………………
Why?
6) Which is the least popular? Why?.........................................................
7) Are there any solar powered lawn mowers in your stock?
………………………………………………………………………………………………………
8) What is the cost of the solar powered lawn mower?....................................................
Are they cheaper to maintain?........................................................................................
9) Do you encounter any mechanical problems with the lawn mowers you stock?
………………………………………………………………………………………………………
10) What are the most common problems encountered?...................................................................
11) Do you get environment pollution as a major problem?.............................................................
12) What steps have you taken to reduce pollution problem?
………………………………………………………………………………………………………
13) Would you be interested in stocking cheaply designed solar powered lawn mowers
soon?..................................................................................................................................
50