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1
SOLAR POWERED ELECTRIC GENERATING SYSTEM
WITH STIRLING ENGINE
Presented to
The Faculty of the Department of Mechanical Engineering
Xavier University
In partial fulfillment of the course
ME 47: ME RESEARCH PROJECT
By
Cabasan, Mark Julius R.
Maraya, Mark Anthony M.
Montante, Rizland C.
Gaoiran, Ash Ton Leo
September 21, 2012
2
Chapter 1
INTRODUCTION
Since the world began people from long ago use the power of the sun for
different purposes, and now in the 21st century people use the power of the sun
basically for energy purposes. The sun is humanity’s oldest energy source, and
scientists and engineers have long sought to harness the power of sunlight for a wide
range of heating, lighting, and industrial tasks. Philippines is geographically located
near the equator where in every square meter of the surface receives approximately 1
kilowatt of thermal energy when the sun is overhead.
In this study, the technical feasibility of developing a solar powered electric
generation technology using a solar thermal collector in conjunction with selected
configuration of Stirling engine. The solar thermal collector receives and concentrates
heat energy from the sun and directs it to a heat receiver of selected engine.
The engine converts moderate temperature to mechanical energy which can be
utilized for electricity generation. In this study of converting solar thermal to mechanical
energy in the process of producing usable electricity while making use of a primitive
engine technology involves searching for a cost effective balance between system
efficiency, and currently available material.
3
STATEMENT OF THE PROBLEM
The research is concerned about the problem of a high cost of electricity and the
shortage of power in Mindanao. Since, large amount of energy from the sun passes
through earth atmosphere and reaches the Earth’s surface, which this energy uses only
in a small percent by human. Some of the energy would be collected through thermal
collector and strike it to the absorbers. The thermal collector would be designed that
would efficiently collect heat energy into the raise of the sun that would enough to
operate the system. The researchers are supposed to have an output in developing a
system that would work for the purpose of generating electricity and most especially an
environmental friendly engine.
The following are the specific questions concentrated in this study:
1. How big is the thermal collector?
2. Based from the temperature recorded, is it efficient to run the engine?
3. What is/are material/s could be used for a good heat transfer of energy with high
system efficiency?
4
OBJECTIVE OF THE STUDY
The main objective of this project study is to develop an engine for power
generation base on the use of solar thermal collector with Stirling engine.
The specific objectives are the following:
1. To identify the appropriate size of the thermal collector to be used.
2. To determine the type of engine to be used that is appropriate for a given
temperature output.
3. To select low cost materials that is good in heat transfer.
SIGNIFICANCE OF THE STUDY
The study of solar powered electric generation technology using a solar thermal
collector that can be helpful to the society. The study aims to convert solar thermal
energy to mechanical energy which can be used to generate electricity at low cost and
at the same time, environment friendly. This maybe small but this can help to lower
electric bills. This can generate electric power using the heat from the sun, which is a
renewable source of energy. There may be devices that already exist similar to the
proposed project but in this study the difference is that it has small space requirement.
The data that can be collected from the study will serve as reference for future
researchers.
5
SCOPE AND LIMITATION OF THE STUDY
The study is focused only in collecting and converting solar energy to mechanical
energy using solar collector and Stirling engine that is to be developed from selected
low cost materials but still offering high system efficiency.
The study is only limited to the conversion of heat energy from the sun to
mechanical power in a manner similar to other mechanical engines. This will involve the
actualization of the project in the laboratory scale. Thus, the study will go through the
fabrication of prototype in order to gather actual parameters that can be used in
calculation of needed dependent variables, tabulation and graphical representation
showing how independent parameters affect dependent variables. In addition, the
selection of material to be used in the solar concentrator is limited to high reflectivity,
low thermal conductivity, low cost, easy to form into a desired shape, and most
importantly commercially available.
THEORITICAL FRAMEWORK
To determine the size of the parabolic dish, a solar constant of about 1.3 kW per
square meter will be used in the equation:
Ad=Qneeded
1.3kWm2
Where; Ad= is the circular area of the 3D parabolic dish
Qneeded= is the power needed to run a selected engine
6
The concentration ratio of a parabolic concentrator (PC) is given by Equation:
CR=AdA r
Where: Ad is the projected area of the concentrator
Ar is the receiver area.
The maximum possible concentration ratio for a collector with a source half angle
of ϴ is given in Equation :
CRmax=1
sinθ2
For a concentrator using the Sun as its source, the half angle is approximately
1/4°. This means the maximum concentration of a concentrator is approximately
5000.The higher the concentration ratio, the higher the maximum temperature will be.
Achieving high temperatures is a unique characteristic to parabolic concentrators. Ide-
ally, the maximum temperature achievable by a concentrator is the source temperature.
The following equation gives the given output temperature for a parabolic concentrator
neglecting conduction and convection.
The rim angle for the concentrator determines the curvature of the dish, a larger
rim angle would result to a steeper slope. The rim angle can be determined knowing the
focal length an collector diameter according to Stine and Harringan (1985) using the
equation below.
ψrim=tan−1( f /d
2 ( fd
)2
−18
¿)¿
Where: f = is the focal length of the collector
D= is the diameter of the collector
7
ψrim= is the rim angle
According to Stine and Harrigan (1985), the rim angle must be determined before
sizing the aperture since it has an influence on the maximum concentration ratio, the
intercept factor, collector slope error and losses due to convection and radiation.
DEFINITION OF TERMS
Stirling Engine – it is an engine which compresses and heats a compressed working
gas and then expanding it in a cylinder which moves a power piston to
produce work.
Solar Thermal Collector –it is a solar collector designed to collect heat which is primarily
composed of metallic sheets formed into wedges, bonded together to
form a parabolic dish.
Reflector- it is used to increase solar collector efficiency by reducing optical and
thermal losses during the reflection of sunlight into a desired point of
concentration
Fins - these are extended surfaces made of highly conductive material
which enhances heat transfer from a surface.
Solar Constant- it is the rate at which solar energy is incident on a surface normal to
the sun’s rays at the outer edge of the atmosphere when the earth is
at its mean distance from the sun.
8
Convection- it is the mode of energy transfer between a solid surface and the adja-
cent gas that is in motion.
Incidence Angle- it is the angle between the ray from the collector to the sun and the
normal of the collector.
Rim Angle- it is an indicator of the curvature for the parabolic receiver
Intercept Factor- it is the fraction of solar radiation reflected from the parabolic concen-
trator that enters the aperture
9
CHAPTER 2
REVIEW OF RELATED LITERATURE
This chapter presents a review of related literature and studies conducted by the
researcher that are significant and related to the problems in developing an engine
through a solar concentrator.
Solar thermal technology is still being developed as an alternative source in
energy generation which makes use of the energy that is released by the sun on the
earth’s atmosphere. Concentration of this energy is one of the key factors to be
considered with this study along with the conversion of this energy into useful electrical
energy. An engine that has been in existence for a century will be modified and used to
convert solar thermal energy to mechanical energy.
The study of solar thermal energy (esolar, Abengoa solar Et.al) states that the
solar thermal electric energy generation concentrates the light from the sun to create
heat, and that heat is used to run a heat engine, which turns a generator to make elec-
tricity. The working fluid that is heated by the concentrated sunlight can be a liquid or a
gas.
…, the concentration of sunlight via a focusing lens achieves temperatures high enough
to ignite paper (approx. 230 °C). Developments in reflector technology for solar thermal
collectors have proven cost-effective for producing moderate to high temperatures... A
10
stationary Compound Parabolic Concentrator reflector can concentrate both beam and
diffuse sunlight within a range of acceptance angles. The theoretical maximum concen-
tration, Ctheory, of a trough collector is equivalent to the inverse of sin (ϴa), where θa is
the maximum acceptance half-angle. The design of CPCs for stationary solar collectors
involves a tradeoff between the concentration ratio and the amount of time during which
sunlight of sufficient intensity falls within the acceptance angle (Konrad H. Aschenbach,
2003).
According to MAIER Christoph, GIL Arnaud Et.al that Beta configuration is the
classic Stirling engine configuration and has enjoyed popularity from its inception until
today. He also discussed several reasons why there is high regard for using Stirling
engine above any other type of engine. One reason is that for this kind of engine it’s
almost impossible to explode. You don’t have to produce steam in a high pressure
boiler. And inside the cylinder there are no explosions needed to run the pistons like in
an Otto or Diesel engine. There are no ignitions, no carburetion because you only need
one kind of gas and no valve train because there are no valves. Finally, it was much
less dangerous to work next to a Stirling engine than to a common steam engine.
The combination of a parabolic dish collector for collecting the thermal heat of the
sun and the high efficiency and dependability of a Stirling engine for the conversion of
heat to mechanical energy makes it ideal for a laboratory scale setup for collecting the
needed data.
The parabolic concentrator must be sized to deliver about four times more ther-
mal energy than the rated electrical output due to an average net system efficiency of
around 25 % (Diver et al, 2001). Existing Stirling dish systems have been built to pro-
11
vide 10 kW and 25 kW electric with an approximate diameter of the parabolic dish being
7.5 and 11 meters respectively (WGAssociates, 2006).
Chapter 3
RESEARCH METHODOLOGY
Research Design
In this research, quantitative data collection method will be used. Using this
method the relationship between variables will be determined. The independent variable
is the size of the solar concentrator. During test, size of concentrator will be set to
determined sizes. The dependent variables are the supplied temperature (temperature
of concentrated solar radiation) and the speed of the selected engine configuration.
After this research a graphical representation of the relationship between concentrator
size and temperature, concentrator size and speed and temperature and speed will be
presented.
Instruments/Tools
The instruments or tools to be used in this research are thermometer or
thermocouple, tape measure and tachometer.
Solar thermal is the source of power for the desired system so it is important to
know the variation of its temperature using thermometer or thermocouple.
Size of the concentrator is an important variable in this research because all
other variables are dependent on this variable. To measure the size of the concentrator
to be used the group needs a tape measure or any length measuring devices.
One variable to be determined in this research is the speed of the shaft rotation.
12
To know how the speed changes with the other variable the group will be using a
tachometer or any angular speed measuring devices.
The Research Flow
The study aims to develop a solar energy utilizing engine for power
generation with the use of solar concentrator.
Research and Information Gathering
Determination of concentrator size by calculation and selection of
geometry
Conceptual System Design
Fabrication of Laboratory Scale Solar Collector
Temperature Profile Data Collection
Selection of Stirling Engine Configuration
Project Testing: Temperature Vs. Speed Project Modification
Final System
13
Research and Data Gathering
The primary source of needed initial information will be through related literatures
published in internet, books and magazines. Researchers will use relevant information
and data of previous experiments conducted on solar thermal profile, solar collectors
and Stirling engine, all other relevant information will be acknowledged used for initial
calculations. Professional recommendation is also highly taken into consideration in the
fulfillment of the study.
Using the tools mentioned previously, needed data can be collected. Since the
study will be conducted only on the month between November 2012 and February
2013, the solar thermal temperature that can be measured is limited only during the said
months. To widen the scope of data collection the group will rely on the recorded
temperature of the past year/s. This recorded data can be requested from weather
monitoring agencies like PAGASA.
Determination of Concentrator Size by Calculation and Selection of Geometry
Solar concentrator should supply enough heat to start the operation of the
desired system. The solar thermal energy that can be concentrated to achieve the
needed temperature is dependent on the weather and the geometry of the collector.
Therefore, it is important to compute/calculate size and select the geometry of the solar
14
concentrator that would yield the maximum concentration of solar heat. The
experimental set up neglects the use of a diurnal tracking device, therefore the
concentrators’ geometry must be able to accept and concentrate the heat to its focal
point without much adjustment. The parabolic concentrator is selected among other
alternatives because parabolic concentrator is a cost-effective way to achieve
high temperatures via concentration of sunlight. It mirrors and focuses an
image of the sun onto their focal target and can concentrate both beam and
diffuse sunlight within a range of acceptance angles.
In connection, parameters to be determined for the parabolic concentrator are
focal length, the rim angle between the focal point and the edge of the dish, the vertical
height of the reflector and the diameter. The most important parameter is the solar
concentrator’s diameter because the solar thermal energy that can be concentrated is
dependent on its size.
Diameter of the concentrator to be used in this project will depend on the needed
temperature and the available temperature.
Other parameters can be calculated using equations for parabola.
Reflective material to be used for concentrator should have high reflectivity,
should be locally available and at low price.
15
Conceptual System Design
The experimental concept design will be composed of a parabolic concentrator, a
selected configuration of Stirling engine, electric generator, rechargeable battery, and a
lamp for testing the electricity output of the system.
The parabolic concentrator reflects solar thermal energy to a central receiver of
the Stirling engine that is connected to an electric generator. The receiver produces
gradually increasing heat caused by solar thermal heat that pushes the power piston
due to expansion of gas when pushed to the hot end of the cylinder. When air is pushed
to the cold end of the cylinder it contracts and the momentum of the machine, enhanced
Parabolic Concentrator
Stirling EngineElectric Generator
Rechargeable Battery
Lamp
16
by a flywheel which is connected to the generator by belt, pushes the power piston the
other way to compress the air. The belt pulley configuration on the flywheel and the
shaft of the generator produces electrical energy which will be stored into a connected
rechargeable battery. The purpose of the lamp is to test the produced electricity that is
being stored in the battery.
Fabrication of Laboratory Scale Solar Collector
Frame and Receiver Assembly
17
The parabolic dish is to be mounted on a horizontal mount system that has two
axes of movement for easy adjustment when tracking the east to west movement of the
sun during the collection of experimental data. The concept design as shown above is
made up of L bracket steel and round bars that is welded and bolted together to make a
stable frame for the parabolic dish. The control mechanism for the adjustment of the
dish angle is controlled by a simple gear adjustments designed to lock manually when
the desired orientation is selected. A pivot point just below the gearing system is
configured to adjust the rotation of the parabolic dish collector. The receiver assembly
where the Stirling engine to be mounted is welded to the frame to provide a stable
support for the engine that is to mounted on the flat plate frame place in the focal point
of concentration of solar heat.
Temperature Profile Data Collection
Location of the study’s data gathering will be positioned at the 6th floor of the
Engineering building, Xavier University, Corrales Street Campus, for purposes of
convenience in data gathering in terms of instrumentation to be used and hourly
collection of temperature profile for the selected days.
Temperature profile for experimental data to be gathered includes the hourly
monitoring of the maximum temperature reached using the thermometer or
thermocouple. Also included in the data is the east to west angle of elevation. This data
is experimentally relevant in a way that the diurnal east to west movement is caused by
the rotation of the earth therefore the east to west angle of the sun is a linear function of
18
time. Although parabolic concentrator is designed to be stationary it is vital to determine
the range of acceptance angles in order for the collector to operate at maximum
efficiency with regard to beaming and diffusing sunlight into the Stirling Engine. The
data collected will be tabulated using the table below.
DateAmbient
TempClouds
Time of Day
Angle of Elevation
Initial Temp
Maximum Temp
Time Duration to reach
max temp
These data gathered will be the basis for choosing the suitable Stirling engine
configuration to be used that would maximize the generated maximum temperature of
the collector.
Selection of Stirling Engine Configuration
Construction options of Stirling engines should to be chosen. The weight and
relatively low power density ratio are the main disadvantage of Stirling engines in
comparison to commonly used combustion engines. In the past research many different
constructions were examined. Eventually, the Stirling engines with double acting pistons
seem to have the highest power density ratio. In double acting pistons both sides of
pistons are active in work and power generation. In present constructions of Stirling
engines four cylinders are usually used. Work of each of four double acting pistons
should be displaced 90 angle degrees against each other.
19
The thermodynamic properties of working gas of Stirling engines have the
biggest influence of possibility to achieve high energetic efficiency. Fast exchange of
heat is the main factor of working gas selection. Some construction options can
increase or decrease speed of heat exchange. The highest pressure of working gas
increases the speed of heat exchange, but in every constructional option the properties
of the working gas are of great importance. The gas heat capacity is the most important
factor. From all the gases hydrogen has the highest heat capacity. But hydrogen is
dangerous because the possibility of explosion and burning in the air is very high.
Hydrogen makes a relatively wide range of explosion mixture in air between 4% and
74%. The mixture can explode by spark, heat or sunlight. Helium is much more
expensive than hydrogen, but helium has very low chemical reactivity and is included to
the noble gases. It means that helium contributes to burning and explosive safety.
Practical factors often decide the usage of air as the main working gas of Stirling
engines. The set-up of the study is purely laboratory scale that is why air is chosen to
be the main working gas for the chosen Stirling configuration. Leakage in the cylinder is
not a setback in the experimental stage due to abundance of air in the atmosphere. As
mentioned above, the latter working gases are expensive and difficult to handle. Air,
although is 14 times below the heating capacity of hydrogen, 5 times worse than helium,
it compensates for the safety, handling and cost.
After the selection of a working gas for Stirling engines pressure ought to be
chosen. High pressure is convenient from the thermodynamic point of view. The heat
20
capacity rises proportionally to an increase in pressure of a working gas. The heat
capacity rises two times when quantity decreases two times and pressure increases two
times. But relation to the mass of working gas heat capacity rises only 0.8% of each
MPa. It explains why high pressure of a working gas is often used in Stirling engines.
Constructional difficulties limit the top value of working gas pressure. In relatively cheap
constructions of Stirling engines, often compressed air or nitrogen with pressure below
1 MPa is used. In other units helium below 10 MPa is usually used. Hydrogen or highest
pressures of any working gasses can be seen only in special cases (Piotr Drogosz et
al.).
An elementary Stirling engine consists of an engine piston, an exchanger piston
and three heat exchangers: a cooler, a regenerator and a heater. The piston converts
gas pressure into mechanical power, whereas the exchanger piston is used to move the
working gas between the hot and cold sources. Stirling engines are usually classified
according to its mechanical configuration: the Alpha, Beta and the Gamma
arrangements.
The Alpha configuration has two mechanically linked pistons in separate
cylinders connected in series by the cooler, the regenerator and the heater. The Beta
configuration corresponds to the classic Stirling engine, having a power/compression
piston arranged within a single cylinder with a displacer/expansion piston, both
connected to the same shaft in a rather complex manner. The existence of a displacer
aims to move the working gas between the expansion and the compression spaces at
constant volume. Similar to what happens in the Beta configuration, Gamma engines
use displacer-piston arrangements. The main difference between these two
21
configurations is that, in the Gamma engine, the power piston is mounted in a separate
cylinder alongside the displacer-piston cylinder and the working gas can flow freely
between them. This configuration produces a lower compression ratio, but allows an
easier mechanical linkage between the pistons and a convenient separation between
the heat exchangers which are associated to the displacer cylinder and the
compression work space associated with the power piston.
Project Testing: temperature vs. speed
TABLE:TRIAL NO.
AMBIENT TEMPERATURE CONCENTRATED TEMPERATURE RPM
The results are shown in the table and graph the relation of the temperature and
the speed. The speed will vary according to the temperature produced by the collector.
The highest temperature can be attain is during noon time. High temperatures will
results to higher speed of the engine. The graph shows the performance of the engine
as the temperature begins to change. It is expected that the speed is dependent on the
temperature. Until such time, the graph will reach the peak point and we can say that
22
the maximum temperature is reach as well as the speed. We are to consider the peak
point as the basis of the design.
Project Modification
The efficient the system designs the better. Modification of the system should be
done if the group can find a possibility to improve the system efficiency. Increasing
system efficiency can be done by minimizing system losses, improving performances of
every part of the system and adding some Stirling engine accessories.
System losses can lower the efficiency of the desired system. Some possible
losses are leaks, frictional losses between cylinder and piston, between bearing and
flywheel shaft, such losses can be minimized.
Increasing the system efficiency can be done also by improving the
performances of some system part. Heat transfer is the driving force of any Stirling
engine thus increasing the heat transfer capability of the hot and cold part of the engine
will cause an increase of the system efficiency.
Addition of engine accessories can increase the system efficiency. Regenerator
and reflector are accessories that can be added to the system to increase its efficiency.
A regenerator is an internal heat exchanger placed between the hot and cold spaces
such that the working fluid passes through it first in one direction then the other. Its
function is to retain within the system that heat which would otherwise be rejected. A
reflector can increase the solar thermal energy that can be concentrated by reflecting
more solar thermal energy to the concentrator.
In modifying the system, cost should also be considered. Modification should be
cost effective.
23
Final System
The system conceived from meticulous calculation, careful gathering of
experimental data and selection of appropriate materials that would yield high system
efficiency for a laboratory scale setup will then be used as a benchmark for household
scale system that would initially power selected household equipment in future
research.