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Literature Review Report
This was a project that was carried out by the previous student. The aim of this project is to improve the existing design of the sterling engine by enhancing the performance and investigating any modifications needed to be made to achieve this. It was stated that energy consumptions have increased rapidly up to 52% in the past 30 years with the industrialization of developing countries. The reason for behind this is the increase in population and enhancement of standard of living. Fossil fuels not only causes harm to the environment but is it also a limited source (DepartmentofEnergyandClimateChangeDECC2013). Therefore, renewable energy has been in high demand and it causes less pollution to the environment. It has been predicted that the increase in demand for fossil fuels will be stopped at 2020 and solar energy alone will be expected to produce 29% (IanJohnston2017). Therefore, solar Stirling engine will be one of the solutions to increase the source of energy and to help people in developing countries that are in shortage.
The first Stirling engine was patented by Robert Stirling, a Scottish minister in 1816(Urieli1984). The engine operates by obeying the Stirling cycle where there is a thermodynamic cycle that consists of two constant pressure and volume cycles (i.e. Isochoric and isothermal). The engine consists of a heat cylinders, pistons, dispenser (accompanies the movement of the gas in the cylinders. It is designed with a smaller diameter than the cylinder to allow the gas to travel in between the cylinder and the dispenser) and a regenerator (a wire mess to retain heat to improve the efficiency of the engine.) The Stirling engine is an external combustion engine, therefore renewable heat sources can be used such as solar power and biomass. Furthermore, it has advantages, working with less noise and high thermal efficiency since it is externally adiabatic (no heat loss) achieving the Carnot requirement for maximum efficiency, and consistency of power output. Ambient air, helium, and hydrogen can be used as the working fluid of the Stirling engine. (Singh2018)
The Stirling engine consists of three types of mechanism alpha, beta, and gamma. The alpha engine consists of two cylinders and piston for hot and cold heat transfer. It has a high power to volume ratio. However, it consists of problems such as gas leakage in the hot cylinder due to the durability of the sealing. Beta Stirling engine has a cylinder that includes the piston and the dispenser, compare to alpha it avoids the leakage of gas. Gamma is like beta, however, the piston is placed in a different cylinder which is connected to the cylinder of the dispenser. The gas moves between them in a single body. The attachment of the cylinders is mechanically simpler and is used in multi-cylinder Stirling engines. Out of these configurations, it is analysed that gamma has the highest theoretical efficiency. (Stirling2012)
The objective of the project.
- Understand the operation of the Stirling engine.
- Study the of thermodynamic theories.
- Design and build the prototype
- Testing and collecting data.
- Investigate methods to improve the performance of the sterling engine.
In this paper, the performance of the beta Stirling engine is investigated by using a heat source of halogen lamps to replicate to solar energy. Two different powered lamps were used 400w and 1000w. The working fluid used was helium. In the experiment, the pressure inputs were varied from 1 bar to 5 bars. The temperature was measured using an infrared thermometer, where it was recorded that 400 W and 1000w was 623k and 823k respectively. The cold end of the piston was kept at a constant temperature of 300k. To obtain the theoretical results of heat transfer coefficient, nodal analysis was used, however, assuming the transfer coefficient in beforehand is difficult. This was one reason for using lamps to obtain more stable working conditions and distribution of heat compared to a solar energy test. Nodal analysis results were compared with the experiment results.
The dispenser of the beta engine has been altered to capture the reflected ray of the sun. The components are divided into two sections. The upper section is made from aluminum which functions as a heater where the solar energy is absorbed. The bottom section act as a cooler and is made of non-brass ASTR steel. The working fluid was tested with a pressure of 5.5 bar to check any leakage. Figure 1 Beta Stirling engine
In the discussion, it was found that high torque was gained at low speeds of the engine, due to a decrease of flow loss which allowed the heat exchanges to maintain a better thermodynamic cycle. To obtain high torque with high speed, some factors of the components needs to be altered, such as increasing the inner surface area for more volume for expansion and improving the higher rate of heat transfer. Furthermore, as the engine speed increases the power increases, however at a certain point the power decreased rapidly. This is due to the insufficient time for the heat to transfer in the fluid. Also, the mechanical losses such as friction, increase with the speed of the engine. Therefore, it was found to obtain the maximum efficacy of the Stirling engine there is an ideal speed. Moving on the pressure, the power increase with pressure however at a certain point it starts to decrease, this can be due to limited space in the engine and courses sealing problem(Costea1999). The increase in temperature increases thermal efficiency. It was observed that the thermal efficiency was 9.26% and 12.85% for 400w and 1000w in pressure of 5 bar. Finally, to conclude the maximum thermal efficiency of the experiment and the theoretical value was 12.717% at 405 rpm and 25.38% at 237 rpm respectively. The difference is explained due to the low speed of the engine. As the speed increases the difference decreases. As mentioned earlier the increase of speed decreases the rate of heat transfer. In the experiment, they haven’t used a dish to test the Stirling engine. Using a parabolic dish would make the results more reliable for the dish solar sterling engine since heat loosed from the dish and the absorber are involved. (Aksoy2015)
To investigate the overall thermal efficiency of the solar dish Stirling engine a mathematical equilibrium theory is used called finite time thermodynamics (ANDERSON1999). The theory includes finite – rate heat transfer, regenerative heat losses, conductive thermal bringing losses and finite regeneration process time. Furthermore, the theory investigates the effect of the absorber temperature, concentration ratio on the thermal efficiency; the heat transfer between the absorber and the working fluid, and the convection heat transfer between the heat sink and the working fluid.
The setup of the system consists of a dish (which has a dish concentrator and a thermal absorber.) The Stirling engine is located at the focal point of the dish. The dish tracks the sun and reflects the solar energy to the absorber where the heat is transfer to the Stirling engine, which forms the solar-powered Stirling engine. Figure 2 Solar dish receiver
In the experiment it was found that the thermal efficiency decreases swiftly with the increase of the absorber temperature (which increases with the concentration ratio), the thermal efficiency is limited by the optical efficiency of the concentrator. It was found that the optimal temperature for the absorber varies from 1100k to 1300k, this is also where the maximum thermal efficiency of the Stirling engine is obtained which is 34% and was stated that is it similar to Carnot efficiency at about 50%. In the investigation, the increase of the regenerator effectiveness, proportionally increased and the reduction of the leakage coefficient increased the thermal efficiency of the Stirling engine. This paper provides a theoretical guidance for designing and methods of increases the performance of the Stirling engine. (Yaqi2011)
The performance of the beta Stirling engine was tested using atmospheric pressure and air as the working fluid. The heat source for the experiment was the electric heater. The engine tested was designed to have a piston and a dispenser that was an angle of 90 degree to each other on the fly wheel. The piston was made of cast iron that included graphite to minimise friction and achieve high impact resistance. The cylinder and the dispenser were made of ASTM steel. the cold end of the cylinder was cooled by using a water jacket. Maintaining a temperature of 30 degrees.
The engine had been run under different operating conditions during the process of development. The test was conducted under atmospheric conditions and the temperature of 800 ℃. The speed of the engine and torque was measured using a dynamometer and a digital tachometer. After each measurement, an increment of 100℃ ranged from 800 to 1000 ℃. From the obtained results it was established that maximum engine power was 5.98 W at 208 rpm at the temperature of 1000 ℃. It was also investigated that higher speed, higher the torque which is correlated to output power. However, at a certain speed, it was observed that there had been a decrease in torque or power can be explained due to insufficient heat transfer. (Cinar2005)
The purpose of this project is to improve and test the solar dish Stirling engine. The reason for choosing this project are personal aspects regarding the country of birth (Sri lanka) , how people in rural areas are having trouble due to the lack of energy, such as children do not have electricity to study in the night or people are unable to use motors to pull up the water from the wells unless using buckets or a generator, which is expensive. The other reason is the strong passion for thermodynamics.
The Stirling engine configuration used in the project will be beta. Considering the time factor, beta type Stirling engine would be a convenient design and will cost least compared to other configurations due to its single piston and cylinder. Furthermore, the sealing problems compared to alpha is avoided in beta and in (Aksoy2015) it was concluded that the heat performance of Beta Stirling engine was better than other configurations except for gamma. (Abuelyamen2018) (Singh2018)
- In the project there would include mathematical calculations using software, MATLAB or Engineering Equation Solver (EES), to obtain the theoretical efficiency of the Stirling engine to compare the actual values with the tested data to understand the errors in the experiment. This can help to improve the design for future implementations.
- Further research will be made to investigate methods of calculating the parameters affecting the efficiency of the system (Dish solar Stirling engine).
- The materials used would include aluminum, steel, and nylon. The research will be made to minimise cost and better selection of materials.
- For the design and optimization, solid works or Auto CAD will be used.
The risk in the project would be during testing of the engine. For safety, PPE would be worn.
- Abuelyamen, A. and Ben-Mansour, R. (2018) ‘Energy efficiency comparison of Stirling engine types (α, β, and γ) using detailed CFD modeling’, International Journal of Thermal Sciences. Elsevier, 132(June), pp. 411–423. doi: 10.1016/j.ijthermalsci.2018.06.026.
- Aksoy, F. et al. (2015) ‘Thermal performance of a stirling engine powered by a solar simulator’, Applied Thermal Engineering, 86, pp. 161–167. doi: 10.1016/j.applthermaleng.2015.04.047.
- ANDERSON, B. (1999) ‘Minimizing losses – Tools of finite-time thermodynamics’, Thermodynamic optimization of complex energy systems, 69, pp. 411–420.
- Cheng, C. H., Yang, H. S. and Keong, L. (2013) ‘Theoretical and experimental study of a 300-W beta-type Stirling engine’, Energy. Elsevier Ltd, 59, pp. 590–599. doi: 10.1016/j.energy.2013.06.060.
- Cinar, C. et al. (2005) ‘Beta-type Stirling engine operating at atmospheric pressure’, Applied Energy, 81(4), pp. 351–357. doi: 10.1016/j.apenergy.2004.08.004.
- Costea, M., Petrescu, S. and Harman, C. (1999) ‘Effect of irreversibilities on solar Stirling engine cycle performance’, Energy Conversion and Management, 40(15), pp. 1723–1731. doi: 10.1016/S0196-8904(99)00065-5.
- Department of Energy and Climate Change (DECC) (2013) ‘Energy Consumption in the UK ( 2013 )’, pp. 1–9.
- Ian Johnston (2017) Global fossil fuel demand set to fall from 2020, three centuries after the dawn of the Industrial Revolution | The Independent. Available at: https://www.independent.co.uk/environment/coal-oil-demand-renewable-energy-solar-panels-electric-vehicles-investors-a7557756.html (Accessed: 28 October 2018).
- Singh, U. R. and Kumar, A. (2018) ‘Review on solar Stirling engine: Development and performance’, Thermal Science and Engineering Progress. Elsevier, 8(July), pp. 244–256. doi: 10.1016/j.tsep.2018.08.016.
- Stirling, R. (2012) ‘Stirling engine : Wikis’, pp. 1–21.
- Urieli, I. and Berchowitz, D. M. (1984) Stirling Cycle Engine Analysis. Edited by P. H. Proferssor Lipaman. Adam Hilger Ltd.
- Yaqi, L., Yaling, H. and Weiwei, W. (2011) ‘Optimization of solar-powered Stirling heat engine with finite-time thermodynamics’, Renewable Energy. Elsevier Ltd, 36(1), pp. 421–427. doi: 10.1016/j.renene.2010.06.037.
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