STIRLING ENGINE RUN BY UNCONVENTIONAL FUEL/BIOMASS

Stirling engine, first patented by Robert Stirling in 1816, is one of mechanical devices that convert heat from multi-fuels to be useful work. Stirling engine components are less than that of internal combustion engine therefore its simplicity makes this engine friendly usage and maintenance. The increasing of fossil energy demand impacts on natural resources and leads us facing fuel price crisis. Therefore, alternative and sustainable energy which is environmental safe is required. Accordingly, local and harmless resource would be the first choice for energy selection to employ with suitable applications and technologies. Thailand has good potential energies especially biomass from agricultural residues. Stirling engine is an optional and interesting machine for these biomass energies because of its simple construction, quiet operation, no internal combustion and emission. Therefore, Stirling engines were designed, manufactured and tested with many features and driving mechanisms.

Stirling CycleStirling engine is based on Stirling cycle thermodynamically. The internal circulation of the working gas under expansion and extraction in hot and cold spaces, respectively, moves two pistons called displacer and power piston. The schematic diagrams of P-v and T-s diagrams are as shown. 

The net work produced by the closed ideal Stirling cycle is represented by the area 1-2-3-4 on the P-v diagram.  From the first law of thermodynamics the net work output must equal the net heat input represented by the area 1-2-3-4 on the T-s diagram. Process 1-2 : Isothermal compression      Heat rejection to low temperature heat sink      1Q2 = area 1-2-b-a on T-s diagram      Work is done on the working fluid (energy exchange from flywheel)       1W2 = area 1-2-b-a on P-v diagram Process 2-3 : Isochoric (constant volume) heat addition      Heat addition (energy exchange from regenerator)        2Q3 = area 2-3-c-b on T-s diagram      No work is done        1W2 = 0 Process 3-4 : Isothermal expansion      Heat addition from high temperature heat sink        3Q4 = area 3-4-d-c on T-s diagram      Work is done by the working fluid (energy exchange to flywheel)        3W4 = area 3-4-a-b on P-v diagram Process 4-1 : Isochoric heat rejection      Heat rejection (energy exchange to regenerator)        4Q1 = area 1-4-d-a on T-s diagram      No work is done        4W1 = 0

Characteristics of the Gamma Stirling Engine The Gamma Stirling engine is similar to the Beta it that it utilizes the same type of moving parts. It has one major difference. The Gamma power piston does not share a common cylinder with the displacer. Its design employs two distinct cylinders, a feature evident in Figure . However, the hot and cold workspaces of the displacer cylinder require the addition of a thermal barrier. Therefore, in its simplest form, the Gamma configuration also consists of four reciprocating parts and one rotary part. The Gamma shares the same advantages as the Beta and also holds the potential for being mechanically simpler. Gammas are particularly suited to multicylinder applications.

In the preceding explanation, the reciprocating and rotating part count was always prefaced by the phrase “in simplest form.” The reality of conventional commercial Stirling design seldom if ever adheres to the simplest form. Contemporary engines display a range of mechanisms, some fairly complex, to change linear motion into rotary.

Summary of Stirling Engine Configurations

Certain generalizations can be made from the preceding sections. There is a renewed interest in the Stirling cycle for sustainable and/or environmentally friendly electrical generation. Reciprocating piston-type Stirling engines, particularly the Alpha, the Beta, and the Gamma, have been harnessed in these applications and have been reported to be effective. These engine configurations, in their simplest form, utilize four reciprocating parts and one rotary part (per power cylinder). Actual commercial engines are typically more complex (i.e., have more moving parts per power cylinder).

The Stirling engine operates on 2 basic principles: •If you have a gas at constant volume and the temperature is raised the pressure will increase. •Conversely if you decrease the volume of the gas the pressure and temperature will increase.

Design parameters

First step of engine design, power is the key parameter and operating conditions such as working pressure and engine speed that leads to get engine dimension from the swept volume as the following equations. The mechanic of machinery is then derived for dynamic parts. The power output of Stirling engine can be approximately estimated by a simple equation as in Eq.(1) based on Beale number (NB) concept. NB is selected from graph of the Beale number as the function of heater temperature referred in [2].

Where P is engine power (watt), is mean cycle pressure (bar), f is cycle frequency or engine speed (hertz) and Vo is displacement of power piston (cm3) derived from Eq.(2) where h is the piston stroke (mm).

Thickness of engine cylinder, t, can be found from Eq. (3) when p is the maximum operating pressure, d is bore diameter and is the maximum permissible stress.

Efficiency = ( Wexp - Wcomp ) / (Qexp + Qheat )

The measurements for TH were taken outside the cylinder however the temperature around the displacer is slightly lower than the temperature of the cylinder. As a result the actual TH should be slightly lower than the measured TH. TL is assumed to be room temperature, but this is only true if the all the heat is removed from the cold section of the engine. This would suggest that TL is slightly higher than the recorded value. These variations in the measurement of TH and TL suggests a smaller temperature difference, this changes the ideal thermal efficiencies to be slightly lower than the calculated efficiencies of 34.5% and 44%.

If we refer to the page “the principles”, we are able to show that the efficiency may be expressed according to the temperatures (expressed in Kelvin) of the heat source and of the cold source, according to the following formula:

Efficiency = 1 – Tmean/ Tmax

Bn is the Beale numberWo is the power output of the engine (watts)P is the mean average gas pressure (Pa) or (MPa, if volume is in cm3)V is swept volume of the expansion space (m3) or (cm3, if pressure is in MPa)F is the engine cycle frequency (Hz)

• Advantages :• - The silence of operation

- The high efficiency : it is function of the temperatures of the hot and cold sources.- The multitude of possible “hot sources” - The ecological aptitude to respond to the environmental requirements on air pollution.- Reliability and easy maintenance- An important lifetime because of its “rusticity”.- The very diverse uses because of its autonomy and adaptability to the needs and the different kinds of hot sources (from mW to MW).

• Disadvantages - The ignorance of this type of engine by the general public. Only a few fans know it exists. It is therefore necessary to promote it. - The variety of models prevents standardization and, consequently, lower prices.- The problems of sealing are difficult to solve as soon as one wishes to have high pressures of operation.

We are considering following main points in making this project :

• Design & Operational Elements

• · Must be able to operate using a compact heat source that is safe for indoor

• use.

• · Must be able to operate unassisted after starting for a minimum of 5 minutes

• (except for a controlling heat source).

• · Must be built to a standard which delivers a minimum service life expectancy

• of 5 years, if properly maintained.

• Size, Weight and Complexity

• · Total engine size and weight to be such that safe and easy transportation is

• possible by 1 person.

• · Must be mounted on a compact support structure for stability and safety.

• · Will be designed for ease of maintenance and assembly.

• Aesthetics & Safety

• · High temperature regions must be clearly indicated.

• · Engine cylinder must be equipped with a removable fitting for piston

• inspection and pressure release.

Thank You…