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Abstract
The aim of this project was to design, build, and test a Stirling engine capable of generating between 200-500 watts of electricity. Several designs were studied before settling on an alpha type configuration based around a two-cylinder air compressor. Concentrated solar energy was considered as a potential heat source, but had to be replaced by a propane burner due to insufficient solar exposure during the testing timeframe. The heater, cooler, regenerator, flywheel and piping systems were designed, constructed, and analyzed. Instrumentation was built into the engine to record temperatures throughout the assembly. Several tests were performed on the engine in order to improve its running efficiency, and critical problem areas were isolated and addressed.
SYNOPSIS
. Since the combustion of the Stirling engine is continuous process, it can burn fuel more completely and is able to use all kinds of fuel with any quality. Because of its simple construction, and its manufacture being the same as the reciprocating internal combustion engine, and when produced in a large number of units per year, the Stirling engine would obtain the economy of scale and could be built as a cheap power source for developing countries. For solar electric generation in the range of1–100 kW, the Stirling engine was considered.
Although the Stirling engine efficiency may be low, reliability is high and costs are low. Moreover, simplicity and reliability are keys to a cost effective Stirling solar generator .The objective of this article is to provide a basic background and review of existing literature on solar-powered Stirling engines and low temperature differential Stirling engine technology. A number of Stirling engine configurations and designs, including the engine’s development, are provided and discussed. It is hoped that this article will be useful in discovering feasible solutions that may lead to a preliminary conceptual design of a solar-powered low temperature differential Stirling engine
CHAPTER -2
Introduction
Dr. Robert Stirling developed the true Stirling engine design in 1816 . Stirling’s heat economiser, now known as there generator, drastically improved the efficiency of the closed cycle air engine. The regenerator acts as a heat exchanger between the cold and hot sides of the engine, absorbing heat from the working fluid during the expansion stroke, and returning it during the compression stroke.
Rev. Dr. Robert Stirling
and hot air engines at the time could not compensate for this lost heat. The addition of the regenerator allowed the Stirling engine to enjoy a period of unrivaled efficiency. It was also significantly safer to operate than steam engines, as their boilers ran the risk of exploding. Soon, advances in steam engine design, and later internal combustion, eclipsed the Stirling engine in terms of practicality and efficiency. It became much cheaper to produce high-horsepower steam engines because of advances in materials and boiler construction.
LITERATURE SURVEY
This project was to design, build, and test a stirling engine capable of generating between 200-500 watts of electricity, several designs were studied before settling on an alpha type configuration based around a two-cylinder air compressor. Concentrated solar energy was considered as a potential heat source, but had to be replaced by a propane burner due to insufficient solar exposure during the testing timeframe. The heater, cooler, regenerator, flywheel and piping systems were designed, constructed, and analyzed.
Instrumentation was built into the engine to record temperatures throughout the assembly. Several tests were performed on the engine in order to improve its running efficiency, and critical problem areas were isolated and addressed Explore basic manufacturing techniques by building a sterling engine. The class is concluded by all of the students running their engines at the same time. As the students discover, the sterling engine is very sensitive to manufacturing tolerance, specifically the fit of the components determines both the friction in the engine and air leakage out of the engine.
The purpose of this project was to develop a model of the sterling engine that accurately predicts the effects of leakage and friction on engine performance
3.1External heat supplies the remainder.
1.Process 3-4: Isothermal expansion. The expansion piston is moved by the expanding fluid, which is maintained at a constant temperature by the external heat source. Work is done in this stage on the piston by the working fluid.
to transfer the fluid from the expansion to the compression space.
2.The regenerator absorbs heat from the fluid, reducing the fluid temperature to that at 1’.However, nearly thirty years later Graham Walker was still bemoaning the fact that such terms as 'hot air engine' continued to be used interchangeably with 'Sterling engine' which itself was applied widely and indiscriminately.
3. The situation has now improved somewhat, at least in academic literature, and it is now generally accepted that 'Sterling engine' should refer exclusively to a closed-cycle regenerative heat engine with a permanently gaseous working fluid, where closed-cycle is defined as a thermodynamic system in which the working fluid is permanently contained within the system and regenerative describes the use of a specific type of internal heat exchanger and thermal store.
4.An engine working on the same principle but using a liquid rather than gaseous fluid existed in 1931 and was called the Malone heat engine
5.It follows from the closed cycle operation that the Sterling engine is an external combustion engine that isolates its working fluid from the energy input supplied by an external heat source.
6. There are many possible implementations of the Sterling engine most of which fall into the category of reciprocating piston engine.
NEED FOR AUTOMATION
• One side of the engine is continuously heated while the other side is continuously cooled.
• First, the air moves to the hot side, where it is heated and it expands pushing up on a piston.
• Then the air moves through the regenerator to the cold side, where it cools off and contracts pulling down on the piston.
Temperature change inside the engine produces the pressure change needed to push on the piston and make the engine run
4.1 Theoretically
• Stirling engine efficiency = Carnot efficiency
• Unfortunately working fluid or gas is not ideal this causes the efficiency to be lower than Carnot efficiency.
In fact, Stirling engine efficiency depends on
• Temperature ratio (proportionally)
• Pressure ratio (inversely proportional)
• Specific heat ratio (inversely proportional)
• Power piston – small tightly sealed piston that moves up when the gas inside the engine expands
• Displacer – larger piston and it is very loose in its cylinder so air can move easily between the heated cooled sections of the engine as the displacer moves up and down
• These piston move by the action of compression and expansion.
• Difference in pressure causes the piston to move and produce power.
CONTAINERS
• A container is a basic tool, consisting of any device creating a partially or fully enclosed space that can be used to contain, store, and transport objects or materials.
• In commerce, it includes "any receptacle or enclosure for holding a product used in packaging and shipping.
• " Things kept inside of a container are protected by being inside of its structure.
• The term is most frequently applied to devices made from materials that are durable and at least partly rigid.
5.1.2.PISTON
• A piston is a component of reciprocating engines, reciprocating pump, gas compressors and pneumatic cylinders, among other similar mechanisms.
• It is the moving component that is contained by a cylinder and is made gas-tight by piston rings.
• In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod.
• In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder.
• In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.
5.1.3.CRANK SHAFT
• A crankshaft—related to crank—is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion;
• whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion.
• In order to do the conversion between two motions, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.
• It is typically connected to a flywheel to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a tensional or irrational damper at the opposite end.
5.1.4. HEATER
The operation of a Stirling engine requires that a working fluid in a closed system is both cooled and heated to induce the expansion and compression cycle. The thermal energy that is introduced into the system is done so in the expansion cylinder of the engine, using a heater. The heater in any Stirling engine design must meet several requirements. First is the ability to transfer heat through either itself or the cylinder wall and into the expansion cylinder without significant losses. It must also be able to maintain a closed seal on the working fluid system, and to limit accidental heat transfer into sections of the engine outside the expansion cylinder.
The design of the heater is dependent on the type of Stirling engine. Alpha and Beta type Stirling engines have heating sections that are separate from the rest of the engine body, and thus have some freedom with the heater positioning and design. Gamma engines, since their heating and cooling systems have to be in line with each other, have limited possible designs for their heaters. This information inclined us to focus on an appropriate heater for an alpha type engine. In an alpha type engine the heater is usually positioned either on top of the hot piston so that the air has to flow through the heating area into an insulated expansion cylinder, or along the walls of the expansion cylinder themselves. The difficulty in choosing the orientation of this component comes from having to balance having the highest possible area for heat transfer while also having low dead volume in the system. If the dead volume is minimal but there is almost no heating area then no heat transfer can take place, and if the dead space is too large then the required heat transfer would increase substantially. Since we had chosen to base our engine off a dual piston V-block, we were limited to using a heater that is heating air traveling into the hot side rather than heating the chamber itself. This is due to the compressor being made of cast iron, which has a fairly low thermal conductivity and makes heat transfer across it into the expansion cylinder difficult. Our heater, therefore, had to be positioned on the opening of the expansion cylinder and made of a material with a higher thermal conductivity than cast iron to provide the most heat transfer to air entering the cylinder.
The compressor caps held a one-way valve on the ends of each cylinder chamber. While the initial design, as shown in Figure 12, was external to the compressor cap, a desire to limit dead space led us to replace one of the original caps with the heater. For simplicity, we used the same bolt pattern as the original cap when designing our heater. The preliminary design was a simple extension of the expansion cylinder into an open chamber, with an exit hole on the side
Our redesign accounted for these problems by first reducing the volume of the heater and secondly by replacing the copper plate. The general shape of the heater remained the same except we choose a basic rectangle base instead of the compressor base, which reduced the height of the part to .75 in from 2 in. This change reduced the volume of the part significantly from 18.18 in3 to 7.27 in3. For the heat transfer plate, we selected an aluminum plate with a thickness of 0.125 in. Aluminum alloy 6061 has a lower thermal conductivity than copper, at 180 W/mK compared to 400 W/mK. However, this was still sufficient for our purposes, and in testing it did not warp like copper. This redesign also forced an alteration of the position of the hole for the exit pipe. The reduced height of the part meant that the diameter of the pipe (1.125 in) was too large to exit from the side. We decided to reposition the hole so that the pipe exited normally from the heat transfer plate and then angled 90 degrees away.
Construction of the heater was achieved over several machining operations in which we pocketed out the internal volume of the new cap, bored the opening to the expansion cylinder, and drilled the holes for the piping and bolts. As a final modification to our design after construction had finished we used aluminum foil as sealant between the heat transfer plate and the cap, and around the piping entering the heater. The foil acted as a high temperature resistant form of Teflon tape as most other adhesives and sealants cannot sustain the temperatures encountered during anticipated steady state operation.
5.1.5.Flywheel
The flywheel of a Stirling engine stores some of the mechanical energy generated in the power stroke of the cycle, and returns it to the crankshaft when the pistons reach their full extension. This overcomes the locking up of the pistons and allows for continuous motion within the engine.
The design of the flywheel began with the need to produce a flywheel that can store enough energy to overcome the measured torque required to start the engine. As measured during our tests, the engine required an average of 7.8725 inch pounds of torque. Therefore a flywheel capable of producing more than that amount is required. For practicality of manufacturing, and ease of configuration, we utilized several disks of 6061 aluminum. The disks were bolted together, and another bolt was used to attach the assembly to the output shaft of the engine. The connecting bolt was secured by lock nuts. A small gear was attached to allow for power transmission to the generator.
Two basic equations define how much energy the flywheel imparts to the shaft. For this project, our flywheel weight was 1.66 lbs., with a radius of 2.725 inches (.227 ft.). Our tests brought the flywheel up to a maximum of 1150 RPM. It can be seen that we achieve 26-inch pounds of torque when rotating the flywheel at 1150RPM, giving us more than three times the torque required.
The moment of inertia of the wheel (I) is ? = ?2
Our flywheel can be approximated as a solid cylinder, so k=1/2. Therefore:
The kinetic energy of the flywheel (Ef) is
At the highest RPM value, our flywheel stores more than enough energy to overcome the startup torque requirement, and is capable of returning the pistons from full extension to complete the Stirling cycle.
5.2.Heat Sources
We researched several renewable solutions for providing the heat our engine would require. Our preliminary list included 13 possible heat sources we would potentially tap into. After weighing the pros and cons of each, we decided the Fresnel lens was our best choice. One of the potential heat sources we considered was the parabolic dish. The parabolic dish has the potential to reach temperatures over 200 degrees Celsius. It is also versatile. Depending on the design of the dish, the size of the focus as well as focal length can be optimized. These are great attributes for a solar collector.
Another possibility was a solar furnace. Similar to the parabolic dish, small-scale versions of the solar furnace can reach over 150 degrees Celsius. While these devices are somewhat commercially available, we were unable to find one that was the right size for our purposes.
The heat produced in a server room was another possibility. While these rooms usually reach temperatures of only 95 degrees Fahrenheit , they produce consistent heat. The consistency is important because the Stirling engine only relies on the differential to produce energy, not necessarily high temperature. This option was eliminated because we did not believe our engine design could generate effective power at those temperatures. We also lacked reliable access to an appropriate testing environment.
The waste heat from a commercial deep fryer was also discussed. At the Frito-Lay Plant in B inghamton, New York, they are able to recover up to 160 degrees Fahrenheit from their frying process. However, this is a low grade heat of varying temperature, and would not maximize the effectiveness of a Stirling engine.
We also considered electronics, which produce waste heat through their own operation. A laptop can produce a large amount of heat while it is running, but this is often inconsistent. The power adapter from a laptop was also discussed because it stays hot almost any time the laptop is on. It also is small, portable, and could potentially be easy to harness. Light bulbs are another product which release a fair amount of consistent heat. They are seen everywhere which could make it a good application for a wide variety of locations. We considered the back of a refrigerator as well because this too would have consistent heat that could be tapped into. While these are each viable heat sources, they would require a low temperature differential engine and would not see a good amount of power removed.
The final route we considered but eventually passed on was geothermal. If it could be harnessed it could be a great heat source due to its consistency and availability. However, for the purposes of this project it would be difficult to tap into and therefore was decided against. After eliminating many of the above options, we decided to use a Fresnel lens as our heat source. This solar concentrator is usually made from an acrylic resin, which allows for a long lifetime and also allows it to be transparent to most wavelengths of light in the solar spectrum. The Fresnel Lens is also cost effective thanks to its cheap manufacturability and has been proven by the test of time as its been used successfully for other purposes for centuries.
PROPANE BURNER
As discussed in the results section, we were unable to generate sufficient energy from our lens during the winter in Worcester. This led us to seek out alternative heat sources to simulate the power of the lens. We decided upon a concentrated propane burner as our testing heat source. We purchased the burner as well as the propane tank hose attachment through Amazon.com. The head of the burner is 3 ¾ inches in diameter and the shaft runs 9 inches long. On the end where the hose is connected there is an adjustable air intake valve to modify the flame once the burner has been lit. We used a standard propane tank to run the burner throughout our engine tests. The burner is made to operate with a maximum capacity of 150,000 BTU/hr. We chose this burner for its higher power output, cost, and its size. This burner head was the closest in size compared to the heater cap.
5.2.3.Generator
After testing the preliminary components of the Stirling engine, our generator was next to be tested. We had to make sure that it was going to be reliable and that it would produce a certain amount of power given a certain RPM input.
When looking at generators to achieve the electrical output from the mechanical rotation of the engine’s shaft we searched for one that would not hinder the engine while it was trying to achieve full speed. We also needed this generator to be cost effective as we only had a little less than $200 to spend. We determined that the best solution to this problem would be to use a generator that is used on smaller wind turbines because when the wind is not blowing as hard then the force turning the shaft of the generator is also much less and the system should still be able to rotate freely. The solution to these problems came in the form of the Windzilla
Permanent Magnet Motor. The motor was roughly $150 and could generate upwards of 250 Watts while having little to no resistance when trying to start the engine.
After receiving the generator, we used a drill to rotate the shaft while using multimeters to measure the voltage and amperage produced. We also had a tachometer pointed at the shaft of the generator so we could tell just how fast the shaft was spinning and correlate the values of amperage and voltage appropriately. The drill was spun at different intervals ranging from 700 RPM to 2300 RPM in order to test the minimum and maximum capabilities of the generator.