Stirling engine works on the principle of heat energy conversion to mechanical energy. Stirling engine is a regenerative type closed-cycle heat engine. It operates by expansion and compression of air or any other working fluid. In closed-cycle type heat engine cycle working fluid is contained inside a cylinder. Stirling engine is an external combustion engine and have greater efficiency compared to the internal combustion engine. Stirling engine does not require any kind of intake or exhaust valve which makes them smooth running, as it is external heat engine it can use any kind of heat from any source like solar, wind, hydrogen etc. One of the significant advantages of using a Stirling engine is that, as they can work on any renewable heat source and does not depends on the fuel or coal their running cost will not rises with rise in the fuel price. As these engines have lower ratio of power-to-weight they are most suitable where weight and space are not constraints.
Stirling engine was first discovered in the year 1816 by scientist Robert Stirling. Stirling engine work mostly at the high range of temperature and also help in using the material efficiently. Stirling engine basically possesses one heat sink which restore all the energy generated and one heat sink which provide all the energy required for working of the Stirling engine. A single Stirling engine can have two or more but up to five heat exchangers in its assembly.
Above figure shows the half portion of the Stirling engine. Point 1 indicates the walls of hot cylinder region while point 2 indicates the walls of cold cylinder region. Point 3 indicates the tubes which working fluid goes inside and outside of the Stirling engine. Point 4 is very important part as this provides the insulations and separates the walls of hot cylinder and cold cylinder to reduce the chances of heat loss. Point 5 and point 6 indicates the piston. Point 5 represents the displacer piston while point 6 represents the power piston. Last point 7, represents the flywheel and crank linkage of the Stirling engine.
Source of heat
Stirling engine can take the heat by combustion of fuel, as it is external combustion engine combustion gases do not mix with the working fluid which flows inside, this provide Stirling cycle some advantages over other type of internal combustion engine. A Stirling engine can take heat from other renewable energy sources like, solar energy, geothermal energy, waste heat energy, nuclear energy etc. Stirling engine powered by solar energy are very popular as this type of engine provide high energy efficiency and also helps in reducing the use of fuel, which makes them cheap and efficient.
In Stirling heat engine regenerator operates as an internal heat exchanger which store the energy released between the hot cylinder and cold cylinder. In this working medium like air or any other gases first takes the heat than passes in one direction than return in the other direction without any mixing of the fluid. Regenerator increases the efficiency of the Stirling engine by reusing the waste heat. This reusing of the waste heat produces more power output which indirectly increases the overall thermal efficiency of the Stirling heat engine.
In Stirling hear engine atmosphere works as a source of heat sink, if larger heat it getting generated, a radiator is utilized which increases the rejection of heat to the surroundings.
Stirling engine flywheel keeps the energy generated during the power stroke and gives it back to crankshaft when piston is at its extreme position. Thus flywheel helps in smooth working of the Stirling engine without any hurdle or stoppage. Stirling engine flywheel is done considering the fact that it can restore the enough energy to start of the engine in case of high torque. Flywheel store the enrgy in the form of kinetic energy which can be represented as,
Where ‘I’ is the moment of inertia of the flywheel while ω is the angular velocity, which can be calculated using the equation,
Where m, r and N are mass, radius and revolution speed of the flywheel.
In Stirling engine air is mostly used as a working fluid, while in some cases helium and hydrogen can also be used as working fluid because they possesses low molecular mass and large thermal conductivity.
Sayyaadi and Ghasemi (2018) conducted modelling of a second order Stirling engine considering the losses due to the crack speed. They studied the effect of rotational speed on the temperature of heat and cold cylinder. They concluded that model developed by them has higher thermal efficiency compared to the previous thermal models developed. Bataineh (2018) numerically studied the alpha-type Stirling engine. They also conducted the parametric study (geometric and operation parameters) to see their effect on the engine performance. They concluded that engine performance can optimized by using the right values of geometric parameters. Lpci and Karabulut (2018) conducted numerical analysis of alpha-type Stirling engine. They conducted thermodynamic and dynamic analysis. Chahartaghi and Sheykhi (2018) conducted exergy and energy analysis of beta-type Stirling engine for different working conditions. They concluded that increment in the rotational speed increases the thermal losses and exergy efficiency reaches maximum. They also concluded that hydrogen gives high exergy efficiency due to the low viscosity.
Ding et al (2018) developed a model to optimize the volume ratio of the Stirling engine. To analysis this they studied the regenerative losses, loss of heat and heat exchanger pressure. They concluded that no functional relationship exists between the thermal efficiency and volume ratio. Sowale et al (2018) conducted analysis of gamma type Stirling engine in a energy recovery system. They studied the effect of working fluid on the Stirling engine performance and temperature of hot and cold region. Cinar et al (2018) conducted manufacturing and testing of an alpha-type Stirling engine. They conducted different sets of experiments and found that with increment in the torque charge pressure increases. Abuelyamen and Mansour (2018) conducted CFD modelling of the alpha, beta and gamma type Stirling engine to analyse their efficiency. They also validated their results with the experimental results available in the literature.
Cascella et al (2018) conducted their analysis to develop the low temperature differential Stirling engine walls. They utilized open source software OpenFOAM for their analysis. They concluded that the reuse of radiative energy loss can improve the efficiency of the Stirling engine. Dai et al (2018) conducted regeneration analysis of Stirling cycle due to the regenerator temperature differences. Hu et al (2018) numerically analysed displacer-coupled multi-stage thermo-acoustic Stirling engine. They concluded that working frequency, mechanical resistance and charging pressure do not influence the power and efficiency of the Stirling engine. Erol et al (2018) conducted review study on development of rhombic mechanism utilized in the Stirling engine. They also studied the effect of the working fluids on the performance of the Stirling engine. They concluded that rhombic mechanism is best for beta-type Stirling engine when compared with the other mechanism. Mohammadi and Jafarian (2018), conducted CFD analysis of hydrodynamics of oscillating flow in beta-type Stirling engine. They utilized the open source software OpenFOAM for their analysis. They utilized SIMPLE and k-w model for investigation of the turbulence.
Figure 4 represents the geometry of the Stirling engine created in the Autodesk while figure 5 represents the exploded view of the entire Stirling engine drawn. In exploded view one can easily see all the parts of the Stirling engine.
Figure: Geometry of the Stirling engine created in Autodesk
Figure: Exploded view of the Stirling engine
Figure: Top, front and right view of the Stirling engine
Stirling engine have two constant volume heat regeneration processes and two constant temperature processes. In one constant volume regeneration process working fluid transferred heat to the thermal storage (sink) while in the other constant volume regeneration process working fluid extracts heat from the thermal storage (source). Figure 2 indicates the temperature-entropy (T-s) and pressure-volume (P-V) diagram of a Stirling heat engine. IT can be seen from the figure that in one constant temperature process addition of heat is done from the external source while in second constant temperature process heat is rejected to the sink. Figure 3 clearly indicates the transfer of heat in the Stirling heat engine.
Figure : Stirling engine T-s and P-V diagram
Figure : Stirling engine working principle
A Stirling heat engine can have the higher efficiency compared to the Diesel engine and Otto engine due to the transfer of heat constant temperature. Stirling engine efficiency is same as that of the efficiency of the Carnot cycle. If TH and TC are the higher temperature and lower temperature respectively, efficiency of the Stirling engine can be written as,
References
Abuelyamen, A., & Mansour R-B., (2018), Energy efficiency comparison of Stirling engine types (α, β, and γ) using detailed CFD modelling, International Journal of Thermal Sciences,132, 411-423.
Bataineh, K.M., (2018), Numerical thermodynamic model of alpha-type Stirling engine, Case Studies in thermal engineering, 12, 104-116.
Cascella, F., Gaboury, S., Sorin, M., & Teyssedou, A., (2018), Proof of concept to recover thermal wastes from aluminum electrolysis cells using Stirling engines, Energy Conversion and Management,172, 497-506.
Chahartaghi, M., & Sheykhi, M., (2018), Energy and exergy analyses of beta-type Stirling engine at different working conditions, Energy Conversion and Management, 169, 279-290.
Cinar, C., Aksoy, F., Solmaz, H., Yilmaz, M., & Uyumaz, A., (2018), Manufacturing and testing of an α-type Stirling engine, Applied Thermal Engineering,130, 1373-1379.
Dai, D.D., Yuan, F., Long, R., Liu Z.C., & Liu W., (2018), Imperfect regeneration analysis of Stirling engine caused by temperature differences in regenerator, Energy Conversion and Management,158, 60-69.
Ding, G.,Chen, W., Zheng, T., Li, Y., & Ji, Y., (2018), Volume ratio optimization of Stirling engine by using an enhanced model, Applied Thermal Engineering,140, 615-621.
Erol, D., YAman H., & Dogan, B., (2018), A review development of rhombic drive mechanism used in the Stirling engines, Renewable and Sustainable Energy Reviews, 78, 1044-1067.
Hu, J.Y., Luo, E.C., Zhang L.M., Chen Y.Y., Wu Z.H., & Gao, B., (2018), Analysis of a displacer-coupled multi-stage thermoacoustic-Stirling engine, Energy,145, 507-514.
Mohammadi, M.A., & Jafarian A., (2018), CFD simulation to investigate hydrodynamics of oscillating flow in a beta-type Stirling engine, Energy, 153, 287-300.
Sayyaadi, H., & Ghasemi, H., (2018), A novel second-order thermal model of Stirling engines with consideration of losses due to the speed of the crack system, Energy Conversion and Management, 168, 505-521.
Lpci, D, & Karabulut, H., (2018), Thermodynamic and dynamic analysis of an alpha type Stirling engine and numerical treatment, Energy Conversion and Management, 169, 34-44.
Sowale, A., Kolios, A.J., Fidalgo, B., Somorin, T., Parker, A., Williams, L., Collins, M., McAdam, E., & Tyrrel, S., (2018), Thermodynamic analysis of a gamma type Stirling engine in an energy recovery system, Energy Conversion and Management,165, 528-540.
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