03-10-2016, 03:15 PM
1457520888-SOLARPOWEREDSTIRLING1455548636231.pdf (Size: 850.39 KB / Downloads: 13)
ABSTRACT:
The performance of Stirling engines meets the demands of the efficient
use of energy and environmental security and therefore they are the subject of
much current interest. Hence, the development and investigation of Stirling
engine have come to the attention of many scientific institutes and commercial
companies. The Stirling engine is both practically and theoretically a significant
device, its practical virtue is simple, reliable and safe which was recognized for
a full century following its invention by Robert Stirling in 1816. The engine
operates on a closed thermodynamic cycle, which is reversible. The objective of
this report is to provide fundamental information and present a detailed review
of the Stirling cycle engine. Stirling engine concept is attached to the solar and
the implementations are reported .Different types of stirling engines, their
working, advantages and disadvantages are presented
INTRODUCTION:
A Stirling cycle machine is a device, which operates on a closed
regenerative thermodynamic cycle, with cyclic compression and expansion of
the working fluid at different temperature levels. The flow is controlled by
volume changes so that there is a net conversion of heat to work or vice versa.
The Stirling engines are frequently called by other names, including hot-air or
hot-gas engines.
Stirling engine is a heat engine operating by cyclic compression and
expansion of air or other gas, the working fluid, at different temperature levels
such that there is a net conversion of heat energy to mechanical work. The
stirling engine is associated with solar power to generate electrical energy since
solar power is abundant source of energy.
3.1 HISTORY OF STIRLING ENGINE:
On September 27, 1816, Robert Stirling applied for a patent for his
Economizer at the Chancery in Edinburgh, Scotland. By trade, Robert Stirling
was actually a minister in the Church of Scotland and he continued to give
services until he was eighty-six years old. But, in his spare time, he built heat
engines in his home workshop. Lord Kelvin, who is best known for his work in
the field of electricity, especially submarine telegraphy, used one of the working
models during some of his university classes.
In 1850, Professor McQuorne Rankine first explained the simple and
elegant dynamics of the engine. Approximately one hundred years later, Rolf
Meijer coined the term “Stirling engine” in order to describe all types of closed
cycle regenerative gas engines.
BASIC STIRLING ENGINE:
The engine is designed so that the working gas is generally compressed in
the colder portion of the engine and expanded in the hotter portion resulting in a
net conversion of heat into work. An internal Regenerative heat exchanger
increases the Stirling engine's thermal efficiency compared to simpler hot air
engines lacking this feature. Key components:
Cut-away diagram of beta configuration
1. Pink – Hot cylinder wall
2. Dark grey – Cold cylinder wall
3. Yellow – Coolant inlet and outlet pipes
4. Dark green – Thermal insulation separating the two
cylinder ends
5. Light green – Displacer piston
6. Dark blue – Power piston
7. Light blue –Linkage crank and flywheel
As a consequence of closed cycle operation, the heat driving a Stirling
engine must be transmitted from a heat source to the working fluid by heat
exchangers and finally to a heat sink. A Stirling engine system has at least one
heat source, one heat sink and up to five heat exchangers.
OPERATION OF STIRLING ENGINE:
Since the Stirling engine is a closed cycle, it contains a fixed mass of gas
called the "working fluid", most commonly air, hydrogen or helium. In normal
operation, the engine is sealed and no gas enters or leaves the engine. The
Stirling engine, like most heat engines, cycles through four main processes:
cooling, compression, heating and expansion. This is accomplished by moving
the gas back and forth between hot and cold heat exchangers, often with a
regenerator between the heater and cooler. The hot heat exchanger is in thermal
contact with an external heat source, such as a fuel burner, and the cold heat
exchanger being in thermal contact with an external heat sink, such as air fins.
A change in gas temperature will cause a corresponding change in gas pressure,
while the motion of the piston causes the gas to be alternately expanded and
compressed.
The gas follows the behaviour described by the gas laws which describe
how a gas' pressure, temperature and volume are related. When the gas is
heated, because it is in a sealed chamber, the pressure rises and this then acts on
the power piston to produce a power stroke. When the gas is cooled the pressure
drops and this means that less work needs to be done by the piston to compress
the gas on the return stroke, thus yielding a net power output.
To summarize, the Stirling engine uses the temperature difference
between its hot end and cold end to establish a cycle of a fixed mass of gas,
heated and expanded, and cooled and compressed, thus converting thermal
energy into mechanical energy. The greater the temperature difference greater
the thermal efficiency.
TYPES OF STIRLING ENGINES:
There are two major types of Stirling engines that are distinguished by the
way they move the air between the hot and cold sides of the cylinder:
1. The two piston alpha type design has pistons in independent cylinders,
and gas is driven between the hot and cold spaces.
2. The displacement type Stirling engines, known as beta and gamma types,
use an insulated mechanical displacer to push the working gas between
the hot and cold sides of the cylinder. The displacer is large enough to
insulate the hot and cold sides of the cylinder thermally and to displace a
large quantity of gas. It must have enough of a gap between the displacer
and the cylinder wall to allow gas to flow around the displacer easily.
5.1 Alpha Stirling:
An alpha Stirling contains two power pistons in separate cylinders,
hot and cold. The hot cylinder is situated inside the high temperature heat
exchanger and the cold cylinder is situated inside the low temperature heat
exchanger. This type of engine has a high power-to-volume ratio but has
technical problems due to the usually high temperature of the hot piston and the
durability of its seals. In practice, this piston usually carries a large insulating
head to move the seals away from the hot zone at the expense of some
additional dead space.
5.2 Beta Stirling:
A beta Stirling has a single power piston arranged within the same
cylinder on the same shaft as a displacer piston. The displacer piston is a loose
fit and does not extract any power from the expanding gas but only serves to
shuttle the working gas between the hot and cold heat exchangers. When the
working gas is pushed to the hot end of the cylinder it expands and pushes the power piston. When it is pushed to the cold end of the cylinder it contracts and
the momentum of the machine, usually enhanced by a flywheel, pushes the
power piston the other way to compress the gas. Unlike the alpha type, the beta
type avoids the technical problems of hot moving seals.
5.3 Gamma Stirling:
A gamma Stirling is simply a beta Stirling in which the power piston is
mounted in a separate cylinder alongside the displacer piston cylinder, but is
still connected to the same flywheel. The gas in the two cylinders can flow
freely between them and remains a single body. This configuration produces a
lower compression ratio but is mechanically simpler and often used in multicylinder
Stirling engines. Gamma type engines have a displacer and power
piston, similar to Beta machines, but in different cylinders. This allows a
convenient complete separation between the heat exchangers associated with
the displacer cylinder and the compression and expansion work space associated
with the piston.
APPLICATIONS:
Applications of the Stirling engine range from heating and cooling to
underwater power systems. A Stirling engine can function in reverse as a heat
pump for heating or cooling. Other uses include: combined heat and power,
solar power generation, Stirling cryocoolers, heat pump, marine engines, and
low temperature difference engines.
The Stirling engine is noted for its high efficiency compared to steam
engines, quiet operation, and the ease with which it can use almost any heat
source. This compatibility with alternative and renewable energy sources has
become increasingly significant.
STIRLING ENGINE WITH SOLAR:
Solar energy is one of the more attractive renewable energy sources that
can be used as an input energy source for heat engines. In fact, 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 a solarpowered
prime mover. The direct conversion of solar power into mechanical
power reduces both the cost and complexity of the prime mover. In theory, the
principal advantages of Stirling engines are their use of an external heat source
and their high efficiency. Stirling engines are able to use solar energy that is a
cheap source of energy.
A solar powered stirling engine was patented by Roelf J. Meijer in 1987.
His invention relates a heat engine, such as a Stirling cycle engine, with a solar
dish collector in order to produce electricity .This apparatus consists of a large
dish aimed at the sun to reflect the rays into the focus point, which is located at
the center of the dish. Solar energy is now collected in the form of heat to fuel a
Stirling cycle engine . which operates by letting heat flow from a hot source to a
cold sink in order to do work. The work output of the stirling cycle is then used
to drive a generator and create electric power.
Placed at the focus of a parabolic mirror a Stirling engine can convert
solar Energy to electricity with efficiency better than non-concentrated
photovoltaic cells.
In 2005 It is created a 1 kW Stirling generator with a solar concentrator,
this was a herald of the coming of a revolutionary solar, nowadays It generates
electricity much more efficiently and economically than Photovoltaic (PV)
systems with technology called concentrated solar power (CPS). Nowadays the
company Infina Applications has development a 3 kW Solar Stirling Product.
By a mirror to focus the sun’s rays on the receiver end of a Stirling
engine. The internal side of the receiver then heats hydrogen gas, which expands. The pressure created by the expanding gas drives a piston, crank shaft,
and drive shaft assembly much like those found in internal combustion engines
but without igniting the gas. The drive shaft is connected to a small electricity
generator.
Generally concentration solar power (CSP) and is significant potential
grid for water pumping or electrification.