DEVELOPMENT OF SOLAR OPERATED STIRLING ENGINE

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Chapter 1- INTRODUCTION

The stock of conventional fuel is limited and also the cost is high. The use of conventional

fuel also increases. So we have to find optional source of energy. The solar energy is

renewable energy and also has benefits like the low running and maintenance cost and also

pollution free. In India 300 days are clear sunny days in every year. India has a potential to

produce 30 MW of energy per sq.km. So it is better to use solar energy as a fuel.

Solar energy is more attractive renewable energy sources that can be used as an input

energy source for heat engines. So, any heat energy source can be used with the Stirling

engine. The solar radiation can be focused onto the displacer hot-end of the Stirling engine,

thereby creating the solar-working prime mover. The direct conversion of solar working

into mechanical working reduced both the cost and complexity of prime mover. According

to theory, the principal advantages of Stirling engines are their use of an external heat

source and high efficiency. Stirling engines are able to use solar energy because of cheap

source of energy. Since during two-thirds of the day, solar energy is not available, solar

hybrids are needed.

Since the combustion of the engine is continuous process in Stirling engine, it can burn fuel

more completely and able to use all kinds of fuel with various quality. According of its

simple construction and its manufacture being make the equal as the reciprocating internal

combustion engine, and when produced in a high number of units per year, the Stirling

engine would be obtain the economy of scale and could be built as a cheap working source

for developing countries. For the range of solar electric generation in 1100 kWe, the

Stirling engine was considered to be the cheapest [1]

. However the efficiency of the Stirling

engine may be low, reliability is high and costs are low. Moreover, simplicity and reliability

are keys to a cost effective solar operated Stirling generator.

A Stirling engine is one of the examples of a broad class of heat engines which are devices

designed to convert thermal energy into mechanical energy. The internal combustion, or

gasoline, engine in an automobile is the example of the heat engine. The gasoline engine

uses the combustion of fuel inside a limited volume, whereas the Stirling engine uses an

external heat source to heat the working volume. The heat source can come from burning

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fossil fuels (such as gasoline), solar energy, decaying plant substance, or whatever is

available[2]

. In fact, all the Stirling engine are requires to operate is a high temperature

difference. It is possible to run a Stirling engine by cooling one part of the engine under the

atmospheric temperature. The gas inside the cylinder of a Stirling engine is not burned or

consumed. So, in compare to the internal combustion engine, the Stirling engine does not

require an exhaust or an intake. If a clean (green) external heat source is used into the

Stirling engine, it can be an ecofriendly alternative to engines that burn and emit

hydrocarbons and other pollutants. Stirling engines also has a benefits which is limits noise

pollution because they do not require intake and exhaust valves which usually are the main

source of engine noise. Though, Stirling engines that would be suitable for automobile use

are highr, heavy, and more costly than conventional internal combustion engines. Besides,

Stirling engines require some time to heat up before they starts and the output of the engine

cant be changed quickly for quick acceleration and deceleration. So Stirling engines have

not yet found use in the automotive industries; they have been used as a submarine engine.

Freshly, there has been a rebirth of interest in Stirling engines as the demand for more fuel

efficient and clean engines continues to increase.

1.1. General principles

Stirling engines are mechanical devices working ideally on the Stirling cycle, or its

modifications, in which compressible fluids, such as air, hydrogen, helium, nitrogen or even

vapors, are used as operational fluids. The Stirling engine offers probability for having high

efficiency engine with less exhaust emissions in parallel with the internal combustion

engine. The earlier Stirling engines were huge and inefficient. However, over a period of

time, a number of new Stirling engine have been developed to improve the faults.

The Stirling engine operates by repeatedly totally a sequence of four steps. Each step in the

sequence is reversible and together they form the Stirling cycle. With help of understand

each of the four steps in the Stirling cycle consider two gas filled cylindrical pistons whose

chambers are connected by a thin tube as pictured in Figure 1.1.1.

The left piston has temperature TH and the right piston has temperature TC In the center of

the tube that connects the two chambers is a wire mesh that will be used to temporarily

store heat as described. For each step in the Stirling cycle of Figure 1.1.1 will be mapped to

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curves on a pressure-volume plot of the Stirling cycle shown in Figure 1.1.2. The four steps

of the perfect Stirling cycle are [3]

:

(1-2) the gas in the engine is expanded at the steady temperature TH. The left piston moves

down and the right piston is fixed. In ordered to maintain a steady temperature the gas must

absorb heat QH from the source (Isothermal expansion - Figure 1.1.1a, path 1-2 in Figure

1.1.2).

(2~3) At steady volume V2, the temperature of the gas is lowered down from TH to TC. The

left piston is compressed and the right piston expanded so the total volume remains fixed.

The hot gas is passed from the left chamber to the right chamber. As the gas passes through

the split tube it delivers heat Q to the wire mesh. (Steady volume heat removal Figure

1.1.1b, path 2-3 in Figure1.1.2)

(3-4) the gas is compressed at steady temperature TC. The right piston is compressed and

the left piston is remains equal. To maintain a steady temperature the gas releases heat QC

to the thermal source at TC. (Isothermal compression Figure 1.1.1c, path 3-4 in Figure

1.1.2)

(4-1) at steady volume V1, the temperature of the gas is increased from TC to TH. The left

piston is expanded and the right piston compressed so that the total volume remains steady.

The cold gas is passed from the right chamber to the left chamber. As the gas passes into

the narrow tube it recovers the heat Q stored in the hot wire mesh. (Steady volume heating

Figure 1.1.1d, path 4-1 in Figure 1.1.2) .

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Fig.1.1.1 Dual piston Stirling engine at four different stages Stirling cycle

Stirling engine at various stages of the Stirling cycle. In any real Stirling engine the

idealized Stirling cycle cannot be made. The four steps are fuzzy together and the cycle on

a PV-diagram appears elliptical. This type of engine has one little sealed piston, called the

working piston, and one higher loose fitting known as displacer piston. The role of the

displacer piston is simply to move, or displace, working gas in the engine back and forth

between a heated lower part and the upper cooled part. In the design pictured in Figure, the

lower plate is heated with the help of flame and the upper plate is cooled with the help of

water or the ambient surroundings. The two pistons are linked together such that their

movements are making 90o out of phase. That is, when the working piston is either at its

maximum or minimum height and moving gently, the displacer piston is at its halfway

point and moving at its higher speed. At position 1 of Figure, the displacer piston is in the

upper cold part which forces the working gas to occupy the hot part and be at temperature

TH. Heat is added pressure to the gas and it expands forcing the working piston to move

upwards (path 1-2 in Figure 1.1.2). At position 2 the working piston is at its maximum

height (the gas has its maximum volume V2) and is moving very gradually approximating

the steady volume path 2~3 in Figure 1.1.2. The displacer, on the other side, is moving into

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the hot part causing the gas to move to the cold part. In this design, the displacer itself plays

the role of the wire mesh of Figure 1.1.1 by momentarily storing energy taken from the gas

as it cools from TH to TC. At position 3, because all of the gas is in the cold part, it contracts

(heat is removed from the gas) causing the working piston to slide down (path 3-4 in Figure

1.1.2). At position 4, the working piston is totally compressed (minimum volume V1) and is

moving gradually. The displacer piston is moving upwards forcing the gas into the hot part.

As the cool gas passes by the di