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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

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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.

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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?

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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.

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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

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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

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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.

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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

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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

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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-

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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.

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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

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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

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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.

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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

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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

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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

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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.

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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

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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

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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

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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.

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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.