In the agriculture industry, lots of generators are needed to power the water pump for irrigation system. It is common to use diesel or electricity to power generators in agriculture operations. There are few significant drawbacks of utilizing these generators. Firstly, burning of the diesel fuel and causing the exhaust gases will lead to air pollution and global warming eventually. The environment issues have been aware by the public nowadays. The system which will cause the environment pollution will be strongly against by the society as well as the agencies in charged. On the other hand, diesel fuel has to be transported to the generators’ location which may be quite a distance over some challenging roads and landscape (Solar Wonders, 2007). Additional resources are needed in order to perform the task that been discuss. In contrast, the electric rate increases and resulting high pumping costs have become a burden for the growers. The costly system has become an issue in the agriculture industry that always been discussed.
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In facts, there are also few types of generators which powered by renewable energy in the market for agriculture irrigation system. With approximately 60% of the earth’s fresh water being used to irrigate crops it is not reasonable to assume that all the farmers in the world grab their buckets and do watering of crops (AGRICULTURE ENERGIES LLC, 2012). Today, there are actually wind energy and solar energy powered water pump being used in agriculture operations. With utilizing the renewable energy to power the system will be able to overcome some of the problems that faced previously. But, most of the people considering installing a renewable energy water pumping system have put off by the expense. Viewing the expense over a period of 10 years, however, gives a better idea of the actual cost. By comparing installation cost, fuel costs, and maintenance costs over 10 years, the renewable type of water pumping system may be an economical choice. In one of the research paper, the 1kW-h of electricity that generates by stirling engine using the solar energy is only at the rate of 7.85 cents (Debnath, 2011).
Solar powered pumping systems are generally more reliable and require less maintenance compare to wind powered pumping system. The nature of challenge is to design the system in such the water pumping can run at maximum. Moreover, a different system which is more efficiency can be developing as to maximize the renewable energy into mechanical work done.
Aims and objectives
The objectives of the project are as follow:
To design a power system which can work perfectly regardless of the weather and environment by utilizing the unlimited solar energy
To construct the power system with the agriculture irrigation system in a more effective way in order to increase system’s efficiencies
To monitor the input and output variables of the system and hence provide amendments
To reduce the probability of pollution
To reduce the usages of non-renewable energies which are not environmentally friendly
Justification of the project
First and foremost, the project will significantly reduce the usage of the non-renewable energy such as diesel fuel and hence help in reducing the probability of pollution. Eventually, it will help to make up the green environment for the public and society. The utilizing of the renewable energy to power the irrigation system has made it cost effectiveness in long term usage while compare to the diesel fuel and electric drive generators. Moreover, the system of transforming the solar energy into work done or output power can be the model to apply on others applications as well.
A stirling engine which invented by Robert Stirling in 1816 will be utilize to generate mechanical work by using the solar energy in this paper. Solar concentrator with sun tracker system is used as well in order to achieve the highest solar energy. Next, the water pump is finally integrate with the stirling engine system and run by the renewable energy. The paper will first discussed the ideas which been carried out by other researchers and hence conclude with choosing the best design to be utilize on the project. The execution phases will explain the procedures that will be carrying out step by step during the construction period.
Firstner and Furstner (2011) have mentioned that the most realistic approach of utilizing the renewable energy is to use solar energy to generate mechanical or electrical power. The two basic principles of converting solar energy into mechanical or electrical power are direct and indirect. The direct method is converting the solar energy directly into electricity whereas the indirect method is converting the solar energy through a channel such as local carrier of energy. Back to the 19th century, steam engine is used to generate electricity but unfortunately there were certain exploitation problems that trouble around (Firstner and Furstner, 2011). The most frequent problems that can be seen are the exploitation of steam boilers and underdeveloped materials. At the same time, the stirling engine is invented by Robert Stirling who is a Scottish minister in 1816 (Aishwarya and DhivyaBharathi, 2011).
2.1 Stirling Engine
Aishwarya and DhivyaBharathi (2011) have pointed out the stirling engine is recognized as a safe engine which will not explode unlike the steam engine usually did in that era.
Figure: Approximate replica of the patent drawings for stirling engine (Source: www.sesusa.org)
The figure above showed the approximate replica of the stirling engine’s patent drawings in the study of Firstner and Furstner (2011). The engine runs in a simple manner which used water vapor air as the medium and act as an external combustion engine. Aishwarya and DhivyaBharathi (2011) noted that the engine relies on the cyclic compression and expansion of air or other gases as the working fluid, the net conversion of heat energy into mechanical work is formed at the different temperature levels. The gases used in the stirling engine does not leave which represents a closed loop system and there are no exhaust valves that vent high pressure gases and no explosions take place. All of these contributions have made the stirling engine very quiet. An external heat source such as burning of biomass or solar energy is needed to run the stirling engine. There will be no combustion takes place within the cylinder of the engine.
Figure: Stirling cycle resulting in motoring action
The general working process of the stirling engine is showed on the figure above which adapted from the study of Aishwarya and DhivyaBharathi (2011). The overall process starts with heating up the air at the bottom of cylinder which indicates as ‘E’ and hence expanding and forcing the piston ‘A’ to move upward. In meantime, the displacer ‘B’ is forced downward to the bottom of cylinder. The hot air that been heated up at the bottom of the cylinder is rushed up to the cold end, which indicates as ‘F’, of the engine through the gap of the displacer ‘B’ and the cylinder. At this time, the hot air begins to contract and pulling the piston ‘A’ downward where the displacer ‘B’ moves upward. The cool air on the top is flowing to the bottom end of the cylinder and hence heated up again by the heat source. The cycle of the process start all over again. These are the general working process for a stirling engine.
In addition, Firstner and Furstner (2011) have also pointed out the advantages and disadvantages of the stirling engine in their paper. The using of external combustion utilizing the bio energy and solar energy will significantly reduce the pollution issues when comparing to burning fossil fuel. Silent and calm operation of the stirling engine is leaded by no combustion within the engine. In contrast, the drawback of the system is it requires higher cost and greater weight per kW than the internal combustion engine.
Figure: The basic outline of resonant engine
On the other hand, the resonant stirling engine usually consists of light piston, heavy piston, recuperator between hot and cold zones, and internal power accumulator which mostly represent by a spring. The process is start with the light piston in the hot zone moves quickly to the left than the working piston in the cooling zone move to the right under the influence of air pressure. Next, the spring energy starts to accumulate when the heavy piston starts moving to the right. The hot air is now transmitted to the cold zone through the recuperator. The heavy piston continues moving to the right by inertia and the gas started to cool off and pressure decreased. Due to the pressure decrease at the cooling zone, the heavy piston is been forced to move to the left. Ultimately, the gas will completely transfer to the hot zone due to the inertia of the heavy piston that moving it to the left. The process is then repeated and the amount of input power and the structure are used to determine the frequency of the motor. Besides that, the straight line motion of the process can be converted into circular movement of the flywheel as well (Firstner and Furstner, 2011).
Figure: The basic outline of the resonant engine with flywheel
2.2 Structural design and parameter research on stirling engine with flywheel
The rotary output motion of the stirling engine is being concerned and several structural design is formed in order to achieve highest efficiency of energy transformation. According to Zhang and Ma (2011), the efficiency of stirling engine is mainly manipulated by its structure and parameters. Figure below is one of the stirling engine’s structural design which gives a rotary output motion that found in the paper of Zhang and Ma (2010).
Figure: The structure diagram of stirling engine
First and foremost, a rhombic driving mechanism is introduced in the paper. The mechanism is unique and unlike other normal rotary output mechanism. It is made of six identical length of connecting rod with two gears meshing. Both the upper and lower levels side links are connected to the displace-piston and power piston respectively and hence produce a 90° phase different movement. By regulating the geometric parameters of the 6 links, the transmission phase of the mechanism is able to change. There are several parameters have to take into consideration for the rhombic driving mechanism. Light weight, high accuracy connecting rod machining, good solidarity and other good conditions are necessary for the mechanism to run smoothly. In mechanical design, the fluent movement of the mechanism is able to achieve based on the light weight and good solidarity as well as ensuring proper tolerances. Moreover, the mechanism has the stiffness of sustain forces, reduces the engine vibration and energy loss to reduce the noise.
On the other hand, the heater and cooling parts of the stirling engine do play a vital role as the system itself is solely rely on the temperature difference of the working fluid. In order to absorb heat as much as possible, Aishwarya and DhivyaBharathi (2011) have introduced solar beam concentrator. The concentrator utilizes the parabolic curve act as a reflector to concentrate the sun ray onto a focal point where the heat absorber is located. It is said to be the most efficient means of collecting solar energy (Aishwarya and DhivyaBharathi, 2011). Furthermore, the reflective petals have to be made of high reflective material and high resistance to corrosion in order to reflect all the sun light towards the focus point and also prevent the petals deteriorate from salt spray and acid rain. Besides that, the heat absorber that located at the focal point of the concentrator is made up of solid aluminum block in order to maximize the thermal transfer. In contrast, Zhang et al (2012) have implemented the thermal storage module that used to store up some of the heated fluid. This is useful when the incident ray of sun light is not sufficient to heat up the working fluid, the thermal storage module will assist in a way to increase the temperature of working fluid for the stirling engine to run. Moreover, Zhang et al (2012) have also employed a secondary reflector that locates near the focal point of the concentrator so that the parallel beams are reflected twice and a better result is achieved. Figure below showed the simulation result that been carried out by Zhang et al (2012). It is significantly showed that there are only a few stray lights being escape from the system after implement a secondary reflector.
Figure: The result of ray tracing after implement a secondary reflector
Apart from these, Faraz (2012) have discussed a few types of solar concentrator system and a comparison table is made as well. Parabolic trough systems and central receiver (solar tower) systems are the solar concentrator systems apart from the solar dish systems. Parabolic trough systems utilize a parabolic through-shaped mirror to reflect the sunlight towards the receiver tubes which consists of working fluid. The fluid is then heated up and hence produces superheated steam to generate the power. In contrast, a field of heliostats that used to track the sunlight is included in central receiver (solar tower) systems. The sunlight is reflected to the receiver that locates at the top of the tower. The working fluid is then heated up and used to generate energy in a steam turbine system.
Figure: Parabolic trough systems
Figure: Central receiver (solar tower)
Figure above showing the parabolic trough system and central receiver (solar tower) that discussed in the paper of Faraz (2012). Moreover, table below showed the comparison table of three different types of solar concentrator systems that study by Hou et al (2009). Based on the characteristics of the solar concentrator system, different type of system can be employed under different circumstances.
Table: Characteristic of three different solar concentrator systems
Back to the study of Aishwarya and DhivyaBharathi (2011), a solar tracking system is necessary for the concentrator to follow the sunlight all day long. There will be more energy to be produced as the concentrator stays aligned with the sun. Based on the research of Debnath et al (2011), the efficiency of the stirling system will be increase gradually with the aid of solar tracker. The yield from solar panel is estimated increased by 30 to 60 percent by using a solar tracker system compare to the stationary system.
Figure: Output of fixed panel and solar tracking system
Figure above showed the results of the output current from PV module using a fixed panel and solar tracking system. The results can be act as a reference when the fixed system and solar tracking system are used on stirling engine. The results of using a tracking system is about 13.33% more than the fixed system under same condition based on the graphically result that showed in figure above. With this data, the energy that collect by the concentrator with tracker system in stirling engine is assumed to be increase.
2.3 Performances of stirling engine
Tan et al (2010) have conducted a performance analysis on the stirling engine at a very low differential temperature. The testing is conducted using a gamma type stirling engine which showed in the figure below.
Figure: Gamma type stirling engine
The experiment is carried by heating up the hot plate till 100 °C and the cold plate is maintain at 30 °C through the normal tap water. The heat source is remove once the temperature of the hot plate reach 100 °C and the angular velocity of the flywheel is noted down at a difference of 5 °C. The results are showed at the figure below which adapted from the paper of Tan et al (2010). Furthermore, the output power and efficiency of the system are also determined by adding the load. The results are then discussed by Tan et al (2010) and the recommendation is also suggested.
Figure: Graph of the relationship between rotational speed and temperature differences
The result has showed that the linear relationship of the rotational speed and temperature differences. The higher the temperature differences, the faster the flywheel turn. Moreover, the minimum temperature differences to operate the stirling engine based on this experiment is 39 °C.
Figure: Output power versus applied load
Figure: Efficiency obtained versus load applied
The results for the output power of stirling engine is obtained based on different loads at a constant temperature difference of 70°C. The output power increased in a form of logarithm trend function against the applied load. On the other hand, the efficiency of the system is also calculated based on the ratio of the output energy to input energy. The maximum efficiency obtained is about 0.05% where the calculated efficiency is 18.76%. Based on that, Tan et al (2010) has addressed the drop of the efficiency is mainly caused by the loss of input energy. After the inspection, the power lost is due to the frictional lost between materials which have low mechanical transmission efficiency.
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On the other hand, Kang et al (2010) have conducted an experiment testing on the same gamma stirling engine but with single and double cylinder. The different configuration of the stirling engine gives a different result on the power as well as the efficiency. The double cylinder stirling engine requires less temperature difference to produce a higher rotational speed as shown in the results that produced by Kang et al (2010).
Figure: Variation in engine heater temperature at various cylinders
2.4 Comparison of stirling engine with other solar system
According to the study of Debnath et al (2011), the most frequent used of solar electric system is Photovoltaic (PV) system and it has an efficiency of roughly 15%. Based on the research that carried by Debnath et al (2011), there are more latest technologies that run higher efficiency that the current system. Stirling engine system is one of it and has the ability to perform better than current design. The interests that make up the stirling engine is its low noise, low emissions, fuel flexibility and backup power provision features. In addition, Debnath et al (2011) has found that the stirling based systems are able to achieve a moderate energy cost savings through using a thermal load strategy.
In order to verify the potential of the stirling engine, a comparison of the stirling engine with the existing solar home systems in terms of Non-Recurring Engineering (NRE) cost, payback period, per unit cost and efficiency has been conducted by Debnath et al (2011). In the report, the study is carried out based on a 1kW solar home system and stirling engine system. The costs of both the solar home system and stirling engine system are Tk. USD 7143 and USD 1634 respectively. The life span of the solar home system is 30 years whereas the stirling engine has an expected life of 60000 hours. Considering the system produces 1kW-h of electrical energy for 8 hours a day, the average electrical energy generates through the entire life of the solar home system and stirling engine is 87600kW-h and 59860kW-h respectively. As a result, the cost of the stirling engine system (USD 7.85 cents per 1kW-h) is much more cheaper than the solar home system (Tk. 5.70 per 1kW-h). Under this result, the stirling engine system is roughly half of the solar home system in term of the cost per 1kW-h. Furthermore, the payback period of both the system is calculated in the study and the results showed that the stirling engine’s payback period is only 7 years which is almost 1/3 of its entire life time. In contrast, the existing solar home system’s payback period is 31.13 years which stretches up to 2/3 of its entire life time.
Figure: Comparison between solar home system with stirling engine system
The figure above is adapted from the study of Debnath et al (2011) which shows the variation of output energy for both the system based on different input energy from sun. Refer to the figure, it is clearly seen that the solar dish stirling engine system has a great performance against the solar home system. This has concluded that the stirling engine system is much cost efficient even though in term of the cost efficiency.
2.5 Variables control on stirling engine
In the study of Howard et al (2010), some control systems are proposed in order to ensure the stirling engine runs safely. First and foremost, the pressure control system is integrated on the stirling engine to control the pressure amount of the working fluid stays at the right level. The supply and dump valve is introduced in which the supply valve is opened to allow the high pressure gas flow into the stirling engine when the pressure indicates low and the dump valve is opened to release the high pressure gas to a low pressure tank when the pressure within the engine is too high. Both of the valves are normally solenoid valves which controlled by pulse width modulated (PWM) technique.
Besides that, Howard et al (2010) have also employed a temperature control loop on the stirling engine in order to maximize the efficiency of the stirling engine. The temperature received has to be controlled as to prevent damaging the receiver material. When the received temperature exceeds the certain limit, the pressure is manipulated in order to maintain the heater temperature within a narrow range. This can be done by open the supply valve to provide extra gases into the system when the temperature received is too high. Conversely, the gases in the stirling engine has to be release if the received temperature is too low. By doing so, the parts of the system are protected and the high efficiency of the system is achieved.
Based on the research that been carried out, the stirling engine will be utilize to power the water pump due to its simplicity which pointed out by Firstner and Furstner (2011). Furthermore, the efficiency of the stirling engine is also discussed by the team of Kang and Tan. The efficiency of the stirling engine is much higher than the existing solar home system that been addressed in the paper of Debnath and his team. Besides that, the working process of the system that been discussed by Aishwarya and DhivyaBharathi (2011) will be tested during the construction phase. The structure design and parameters research that study by Zhang and Ma will be act as a reference during the designing process. The several type of solar concentrator systems, which is one of the main parts of stirling engine, is critically discussed in the Faraz’s paper. With obtaining the ideas from the other researchers, the project is then proceed to the next phase.
First and foremost, designing on the structure of the stirling engine will be appear in the very early stage. Autodesk Inventor Professional 2011 will be one of the designing software that will be utilizes. The several main parts such as solar concentrator and linear to rotary motion conversion mechanism of the stirling engine will design accordingly in order to achieve the objectives. The dimensions of the system will be in a small scale prototype. Firstly, the energy captured from the sunlight is the main concern for this project. In order to absorbing the sunlight throughout the whole day long, solar concentrator system has to be design with an aid of solar tracker system. Several designs of the concentrator system such as parabolic trough system and the solar dish concentrator have to be designed and tested in the day time. During the testing, the temperature of the working fluid that absorbing the heat energy through the system act as the main responding variable. Besides that, the materials for reflecting and absorbing the sun ray do play a vital role. Steel plates and mirror are some of the material that can used to reflect the sun ray whereas the aluminum block can be used to absorb the heat. Both the systems are tested under the same condition and the temperature of the working fluid is determined using thermometer every hour. Based on the results, the best design will be utilize later on the stirling engine.
Next, the mechanism that converts the linear motion to rotary motion is designed in order to power up the water pump. In this stage, the rotational speed of the flywheel act as the main responding variable. The faster the speed of the flywheel is, the faster the water flow rate is. Based on the previous research, the rhombic driving mechanism is introduced. Besides that, the normal type of the linear to rotary mechanism can be build up for testing purpose as well. The friction and energy loss have to be concern as well during designing the mechanism. The rotational speeds obtained from the mechanisms are analyzed. The tachometer instrument is used to determine the rotational speed of the flywheel when it’s turning. The purpose of doing the experiment is to choose the mechanism which able to achieve the higher rotational speed. If the idea rotational speed is not achieved, the brainstorming session will be carrying out in order to generate new ideas for the driving mechanism. Besides that, the conversion of heat energy based on the temperature difference into the linear motion has to be following up as well. The thermodynamic theory that makes the piston move in a linear motion when there is a different in temperature at two sides. The smoothness of the piston in the linear motion will be achieved in order to maximize the rotary output motion.
Once both the concentrator system and driving mechanism are constructed, it means the stirling engine has constructed. A testing will be carry out in order to check the input energy that absorb by the absorber and the output energy which represents by the rotational motion of flywheel in term of speed. Experiment will be carrying out for a week and the data is tabulated. If the results do not meet the objectives, the designing process has to carry out again. In converse, if the results are satisfy, the project will precede to the last part which is also one of the main objectives that is connecting the rotational output to the water pump that used for agriculture irrigation purpose. The water pump will be getting from the market based on the data that been gathered. The rotational output of the stirling engine is the main parameter that used to decide which water pump to use.
When the water pump is chose and purchased, the integration of the stirling engine on the water pump is carrying out. Some of the modifications will be done in order to power up the water pump by the mechanism of the stirling engine. Moreover, the parts on the water pump might need some adjustment such as drilling and polishing that can be carrying out in the manufacture factory. Once the water pump is set up with the stirling engine, it will be run by using the solar energy as input without any load. Necessary checking is then carrying out on the modified water pump to make sure the device is safe to use in real life. The load is then adding to the water pump and the testing is run again. At this time, the several parameters of the water pump will be noted down such as change of head and rotational speed.
Lastly, the contribution of the project will be generating based on the recorded results. The significant improvement of the device and the maximum power that generated by the stirling engine are the main concern. The achievement of the objectives for the project is also depends upon the results. By verifying the results with refer to the aims, the project is considered successful.
Phase 1: Designing of the solar concentrator systems to focus the sunlight
Phase 2: Testing on the constructed systems and choose the best design
Phase 3: Designing of the driving mechanism for stirling engine
Phase 4: Testing is carried out to look for the highest rotational output
Phase 5: Checking the linear motion that form by the temperature differences of fluid
Phase 6: Construct the whole stirling engine system with the driving mechanism
Phase 7: Experiment is carried out to check for the efficiency of the system
Phase 8: If the results fail to meet the objectives, designing process will be done all over again. In converse, the project will proceed to the next stage if the objectives are met.
Phase 9: Choosing the right pump with refer to the data of the stirling engine (rotational speed)
Phase 10: Integrate the stirling engine system onto the purchased water pump
Phase 11: Several modifications on the water pump are conducted in order to fit the stirling engine
Phase 12: Testing of the water pump is performed without load
Phase 13: Checking on the modified water pump as to make sure it runs safely
Phase 14: Testing with load is carried out and the results are tabulated
Phase 15: The contributions of the designed water pump are listed and verify with the objectives to check if the project has constructed successfully
Figure: The phases execution through the entire project
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